Though the Imperial armies in many cases got access to better disciplines (as Divination is absolutely fantastic), Tyranids are the unparalleled kings of psykers currently, being able to bring them in their HQ, EL, and TR slots and in great numbers; in theory, a single FoC can contain seventeen different psykers.
However, not all powers are created equal, and for Tyranids especially many of them can end up being unusable or mediocre. For that reason, it's important to look at the various Tyranid psykers as well as the psychic powers available to them and figure out what choices you'll be making.
Each of the Tyranids has access to three disciplines (Biomancy, Telepathy, and Telekinesis) as well as its "standard" powers. The Tyranid powers have changed very little with the new edition- Catalyst is still excellent, Onslaught is fine, Paroxysm/Life Leech are the only realistic choices for a Tyrant, etc, so all of your lists will pretty much be starting with the same powers as before, but now you have an additional option to swap out at the beginning of each game for something different.
The first question is the most basic: should you do this? For some, it's pretty easy- the Broodlord's powers are rather underwhelming (especially with 6+ Deny that can shut it down) and he has no particularly good reasons to keep them. For most, though, it's a lot harder, so we'll look at them on a case by case basis.
The Tyrant is the first and most obvious on the list, and while it's not quite in the same boat as the Broodlord, it's not far off from him. Paroxysm is, admittedly, pretty excellent- making an enemy WS1 before you charge in can totally shift the course of a battle and unlike Fear, the enemy has very few ways to defend against it. However, with that said the other powers are generally unimpressive, with only Life Leech being able to do anything of note. Because of this, the Tyrant will usually want to trade out for one of the disciplines, all three of which contain excellent choices for him. The only real reason to keep to your basic powers is if you are facing another aggressive melee army that you think the extra debuff can swing a fight when you're already expecting to have access to some of the Biomancy powers.
The Swarmlord is basically identical except that Telepathy is extra-awesome for him, since he can access the Warp Charge 2 powers in it; you should virtually always being taking at least one or three Telepathy powers for him.
Tervigons are the first unit where we have a real dilemma, and that dilemma is Catalyst. Catalyst is a fantastic power for protecting your bugs (despite or in some cases because of the changes) and should always be a consideration. However, Biomancy contains some similar powers (Iron Arm for protecting itself, Endurance for being strictly better)- with the caveat that you are not guaranteed to get them. So power-swapping with the Tervi is always a bit of a gamble, but it's a gamble that can give you some very nice stuff. Tervis generally want to pick from the Biomancy table, although Telepathy has a couple good picks as well. If you're looking to power-swap regularly I would suggest grabbing both Catalyst and Onslaught as upgrade powers, as getting the extra one when you swap is nice and Onslaught is actually quite useful on its own.
Zoanthropes are perhaps the undisputed masters of Tyranid psychic powers, since they have BS4 for the witchfires and get two powers each while coming in squads of three. Along with the ability to arrive anywhere on the field, this gives them quite a bit of flexibility, though being T4 does end up meaning that they are very vulnerable to enemy heavy shooting. Coming with Warp Lance/Blast standard they have a good way to kill heavy tanks and an acceptable Marine-killing gun base, but unless you're facing a double-Land Raider list you probably want to be swapping these out. Telepathy is an extremely strong choice for them, giving you several utility buffs and a good anti-infantry attack, while Biomancy is a lot weaker- it has several awesome powers but also several that do very little, so you're either getting awesome stuff or garbage. Telekinesis has some nice bits in it and walks a bit more of a middle ground, but you still have some powers that will often be "dead."
The Doom of Malan'tai can keep Cataclysm if you're expecting to see some heavy armor (tanks or infantry) and want a way to handle some of them, but more often than not you'll want to swap into Psychic Shriek from the Telepathy discipline, since it's basically just an improved version of his normal aura. If you land wrong or they roll well, aim a second one at them and try again. I feel compelled to point out that taking Biomancy has a small chance of giving you Iron Arm, which makes you Eternal Warrior and thus almost impossible to kill, but with the chances of getting it so low it's really only something you should do for lulz.
Broodlords have very little reason to bother keeping their original powers; Aura of Despair is absolute garbage and Hypnotic Gaze, while not terrible, now contends with Deny rolls to try and shut down an enemy character (though challenges can make this less necessary.) However, his Ballistic Skill of zero is greatly limiting in terms of what he can realistically take, as witchfires of any kind are worthless to him. (Note, however, that beams, maelstroms, and novas all work, since he does not have to roll to hit.) For that reason Telepathy is a bit risky for him (two good, one weak, one unusable, unusable Primaris.) Telekinesis is surprisingly decent (three good, one decent, one unusable, decent Primaris.) Biomancy is very hot and cold (four good, two unusable, unusable Primaris) but is still going to be your best pick a lot of the time. Bottom line: Broodlords may be a cheap and flexible psyker, but you will sometimes roll up results that do absolutely nothing, so be prepared for it.
So most Tyranids benefit most from Biomancy, Telepathy, or a combination of the two, although you can certainly do some good things with Telekinesis as well. Remember you pick powers one at a time, so if you swap things out and end up getting one of the "best" powers from a list, you may want to consider calling it good and moving on to one of the other disciplines in hopes of getting some luck there; similarly, if you get a garbage power you are completely unable to use, you may want to take something "safe" for your second pick to ensure your psychic abilities are not completely wasted.
With that out of the way, let's take a look at the powers themselves and how they stack up.
Biomancy Powers
SMITE: The Primaris power for the discipline, it's the same shitty Marine ability we all know and pretend doesn't exist. The reduction to cover is a minor boon, but four S4 shots (even with AP2) are absolutely nothing to be impressed with. Worse yet, most Tyranids are BS3, meaning you won't even get all that many hits with it. It will be a rare day when you decide to swap into this power.
IRON ARM: Gives you +d3 to your Strength and Toughness and Eternal Warrior- the only way 'Nids get the latter. As a self-buff, it can be limited in applications and for MCs Smash Attacks make the Strength bonus less relevant, but it's still useful in many situations and making yourself T8 or T9 can be absolutely crazy. Remember the majority toughness rules when using Iron Arm, as it can drastically shift the numbers your opponent needs to roll.
ENFEEBLE: One of the stars of the discipline, Enfeeble gives -1 Strength and Toughness to an enemy unit and makes them move as in difficult terrain (which can make bikers and jump troops bonk themselves to death, as usual.) Even a single application of this power can drastically swing a combat and multiple applications can make for some laughable situations (though it can't reduce a model to T0 on its own.) With the added bonus of being a malediction and thus not needing a hit roll or limiting your shooting/assault targets, you're hard-pressed to want more than this.
ENDURANCE: The other big name from Biomancy, Endurance gives Feel No Pain, Relentless, and It Will Not Die. The latter rule, for those not familiar with it, restores a wound (or hull point, oddly) at the end of the controller's turn on a roll of 5+. Though it does its best work on monstrous creatures, who are unlikely to suffer instant death and have multiple wounds to make use of IWND, even little bugs enjoy getting an additional chance to shrug off damage. Relentless, unfortunately, is wasted on our models.
LIFE LEECH: Many have complained that this is a "strictly better" version of Leech Essence, but let's look at the fact: Life Leech is two shots, hits on fours, wounds on 2s, and allows cover but not armor; it can only ever give you one wound back. Leech Essence hits automatically d3 times, wounds on 5s (against Marines, anyways), and allows cover but not armor; it can get you back one wound for each one you deal. Leech Essence is actually superior against T3 targets (averaging one wound before saves compared to .85 for Life Leech) and slightly inferior against Marines (.66 vs .85 expected), but has the advantage of a higher upside potential. Now that we've barged through that nonsense: Life Leech isn't a very good power. You'll take it if you roll it up, but it's not really something you want and its range is short enough you won't often actually get any wounds back from it.
WARP SPEED: Following the stat-altering pattern, this gives the caster d3 to its attacks and initiative as well as Fleet; this is decidedly less good than the others, but still acceptable, as more attacks on your fighting critters is nice and it helps push out your charge range a little bit. And for those that have a hate-on for JotWW, it can significantly boost your odds of passing such a test (for a Tervigon, anyways), so you may rejoice in that. Tervis are definitely the greatest beneficiaries of this power, but hardly the only ones.
HAEMORRHAGE: Yes, spelled like that. Haemo is very weak on its own, causing a single wound on a failed toughness test and getting to "chain" to a new model if it kills something; the fact that it is a focused witchfire is only somewhat a bonus. However, paired with Enfeeble to drop the stats of a unit down to well below their normal levels and potentially cause a cascade of deaths. However, this trick is very risky (it requires casting several powers successfully in sequence, hitting with Haemo, and then getting them to roll particular results), but when it does work it's hilarious. Of course, by the time you've double- or triple-debuffed an enemy unit you could kill them with just about anything anyways, so...
Telekinesis Discipline
ASSAIL: The Primaris power; a S6 beam attack with a range of 18" and the Strikedown rule (halves initiative, move as in difficult.) While not terribly exciting, if Assail had been S7 it probably would have been reasonably solid for wounding some models and taking a hull point off a couple tanks; however, at S6 it's not all that likely to have any real effect (despite the boon of the autohit) and thus not really something you care to take most of the time.
CRUSH: This really feels like it should've been the Primaris, but sure, whatever. Strength 2d6 is pretty good and an AP od d6 is also nice- half your shots are Marine-killers and one third of them are tank-killers. (Well, sort of.) It's actually rather shitty against infantry unless you desperately need to try and kill the nearest guy, as even being a focused witchfire doesn't help much when your range is only 12". So use it to try and bash tanks and you'll do okay, but since you can't get it consistently Tele as a whole loses out a lot.
GATE OF INFINITY: Like the SM power, but a couple of changes. Your 24" is measured from any one member of the unit and you can disappear from melee combat for sure (since it's a blessing.) DSing in general is always a bit risky, but Gate carries a small additional risk if you have a squad, as one of them can randomly vanish- Tyrant with Guard beware. Still, it's a pretty excellent utility power and you won't really every be sad to see it come up.
OBJURATION MECHANICUM: Another anti-tank power, this one inflicting an automatic Haywire hit on everyone in the unit and forcing them to reroll hit/wound rolls of six. Awesome against tanks, awesome against Rending/Tesla models, awesome when you have flying monstrous creatures, awesome for preventing Overwatch losses, and a decent little debuff if you don't need any of those other things. All in all, one of your happiest powers in the discipline.
SHOCKWAVE: A nova (so auto-hits all enemies) with a 12" range that causes d6 pinning S3 hits. Not really very scary to most units, but on a flying critter or Broodlord that can get into the middle of an enemy formation you have the potential to cause some damage. Meh, it could be worse.
TELEKINE DOME: Gives one unit within 12" of you a 5++ against shooting that reflects shots just like Necron Lychguard do. To be honest, the reflection part is really just a gimmick- a 5++ is at least semi-reasonable by itself, though hardly great. It will occasionally cause silly stuff to happen on Overwatch, but isn't terribly relevant. Remember that you always use your best save, whether you want to or not, so (for example) a Zoanthrope can never benefit from this power.
VORTEX OF DOOM: Exactly like the SM power but Warp Charge 2 and thus garbage. Only the Swarmlord can even access this and you never, ever want to roll it up.
Telepathy Discipline
PSYCHIC SHRIEK: The Primaris power. A witchfire that causes wounds equal to 3d6 minus the target's Leadership (which will use the highest value in the squad, as normal) with no armor or cover saves allowed. Although a bit random, this can be absolutely brutal to many units and is a great choice if you're looking to murder non-vehicle units of pretty much any kind. This power alone is one of the best reasons to take Telepathy as a discipline, since most units can simply swap any garbage powers they roll up into this (bar a Broodlord, of course.)
DOMINATE: Forces one unit to make a Leadership test any time it wants to do anything (move, shoot, assault, run.) Although most units tend to be high enough Leadership to pass the test the majority of the time, the random chance for them to completely fail at doing something can be a nice addition and it especially punishes units that want to perform sequences of actions (move into position, take some shots, charge you) in order to be used effectively. Like most of the Telepathy powers it really shines against low-Ld armies like IG, Tau, and Eldar.
MENTAL FORTITUDE: Lets a unit in 24" auto-regroup and makes them Fearless. The Synapse rule makes this power largely worthless, though it can be some help to Genestealers. Its large range also lets it occasionally bolster a unit well outside of your Synapse range and keep them from falling back, although it will do nothing to help their Instinctive Behavior test. For the most part this is garbage and should be swapped out first chance you get.
PUPPET MASTER: A focused witchfire, but one that does no damage- instead, the targeted model takes an immediate shot as if it were under your control. Aside from the roll to hit, there are a couple limiting factors here; in a unit, you generally won't be lucky enough to catch the heavy weapons guy (unless you roll that five or lower on the test), but have the advantage of being able to shoot in any direction (as infantry have 360 sight.) Contrawise, against vehicles you have no worry of getting the wrong one, but its weapons are less likely to be pointing in the direction you need them to be. (One also wonders how such units shoot- are they limited by their movement from the enemy's turn, or do they shoot as if stationary since they did not move during your turn and are treated as being one of your units?) In any case the power is a fairly reasonable one and can be used to devastating effect in some cases to turn an enemy Meltagun against his own transport, etc, but will usually be a lot worse than that. If you roll it, be on the lookout for good targets, as it's something that the enemy will have trouble keeping from giving you opportunities on.
TERRIFY: Another all-star power, this removes the Fearless rule, makes the unit Fear all enemies, and forces an immediate morale check- note that the latter part is not a result of casualties and thus Dangerous tests (or subsequent shooting) can force more of these. With concentrated effort it is quite possible to scare a unit right off the board in a single turn despite the inability to escort them off as you used to. This works best on backfield shooting units (like Broadsides) that are vulnerable to running off in a single movement or against large Fearless units (like Orks or enemy Tyranids) immediately before you charge into combat with them.
INVISIBILITY: One of the Warp Charge 2 powers (and thus only accessible by the Swarmlord), this grants the Shrouded and Stealth rules to a unit, makes the enemy fight them at WS1 and prevents them from using Counterattack against you. (Oddly, they still get to Overwatch.) With its huge improvement to cover saves (4+ in the open, even) and pseudo-Paroxysm, Invisibility is an incredibly powerful ability, though you'll need to find an appropriate target to put it on; a large squad of Gaunts or other tough squad can work. Unlike some other codices it's hard to deliver a real death-bomb with this power, but a front wave of Gargoyles that are all but untouchable is nothing to sneeze at and can ensure that your front line of chargers get stuck in with the enemy and tie them up long enough for your other units to arrive.
HALLUCINATION: The other Swarmlord-specific power, this one isn't nearly as scary as the internet makes it out to be, though it's obviously quite excellent. If it goes through the enemy has equal chances of being pinned automatically (unless they were immune to pinning), standing and doing nothing (not even fighting in CC), or inflicting a single hit per model on their own unit. Though many regard the last one as the scariest, in truth that's only true for certain melee hammers- for Orks or Marines, to name some examples, the damage actually caused will be minimal. The other two results, however, can be excellent for taking the enemy out of the fight for a turn, giving you a chance to deal with something else, and the fact that the power is used during movement means you can cast it, see what happens, and plan your turn accordingly. Note that, as a malediction, the effects last until the end of the enemy turn, so if you roll a "can't do anything" result you can freely assault said unit and fight two rounds of combat with him before having to worry about him swinging back- in many cases this can be sufficient to whittle them down to nothing, especially if they are in cover or have dangerous Overwatch (both normally problematic for 'Nids.)
So Tyranids end up with a fair number of good options in their psychic powers, though lamentably not really any that can consistently deal with vehicles, which is sad. However, there are tons of good options in there for utility in the list.
For those of you eagerly awaiting an update to the Tyranid reviews, please be patient. A lot has changed since 5E and your strategies- if not always units- are going to be very different. One thing I can say: hordes of smaller bugs are much, much more dangerous now and seem like they'll for the backbone of many armies, especially in light of the above psychic powers, but I'm still testing out different possibilities and discussing things with the many other people who are playing with 'Nids. I don't want to venture any kind of strong opinions as to their viability or nonviability at this point, as flyers are simply too much of a deciding factor, but certainly there are options there that are quite strong against everything that you don't have to hit on sixes.
