So, you want to bring some allies along and... hold on just a second.
TYRANIDS, GET THE F*** OUT RIGHT NOW. NOBODY LIKES YOU. GO AWAY AND GO TO HELL AND DIE IN A STINKY DITCH. YOU HAVE NO FRIENDS AND NEVER WILL AND ALSO YOU AREN'T ALLOWED TO PLAY WITH OUR TOYS MOM SAID SO.
Okay, there we go. So, you want to bring some allies along and are wondering what to do; do you take special character and try something gimmicky? Is it about psychic defense? Which army do you ally with? Are Desperate Allies ever worth it? Etc. The sheer scope of allies (every codex in print) makes trying to figure out what you want rather tricky, not to mention considering if you even want them- in most cases it is not a small investment.
So let's go over some of the possibilities and see who has what, who wants what, and what sort of things we can be doing with them.
SO we'll be looking at games in the 1500 to 1999 range here; below that allies start to take up too much of your points to discuss in the same light (though they still can be usable, depending); at 2K and up, where you have access to a second FoC your priorities become very different and I don't really feel like I have a strong enough grasp on that yet to talk about it any.
There are a number of different reasons to take allies, but they boil down to essentially a few different reasons. One is to get access to good troops- many codices, especially older codices, have awful troops or troops that are ill-suited to scoring objectives; for these codices, the ability to take a pair of someone else's troops is a real boon, as that can get you a significant number of scoring bodies in many cases. The second is to get some kind of psychic defense or psychic powers- again, many books simply don't have access to this sort of thing or have very poor access to it- being able to bring in a strong unit, generally in the HQ slot, can fill a gap in their list. Lastly we can bring in a variety of specialist units that do jobs the normal codex can't or can't do efficiently- it's a well-known fact that some books have some amazing choices for certain roles (like Broadsides or Psyflemen for AT) and gaining access to them can really round out an army.
(One final, more niche use is to gain access to additional slots- the "free" HQ, troops, and others can occasionally be useful to armies that quickly fill their own up with efficient choices.)
Looked at from this perspective, things become a lot simpler; we can look to each book and ask ourselves how it fills each of these roles and what unique choices it brings to the table and what they can do for us. So let's do that, codex by codex, and see what we come up with.
Black Templars
Uh-oh, this doesn't look good. BT don't really bring a lot to the table compared to other Marines; they're the same price but with fewer toys in most cases. You can get heavy + special with only five bodies, so you can get a 158pt Rhino/Lascannon/Plasmagun squad, which isn't terrible. Your HQ slot is a lot less useful, however; a budget-level Castellan clocks in at 65pts and isn't awful (two-wound Marine Captain, basically), but you're not getting much of anything out of him except a cheap way to issue/accept challenges. None of the other slots particularly stand out, either, although you can get a Land Speeder with missiles reasonably cheap.
Blood Angels
Blood Angels are blessed with a number of different options for being allies; their HQs have several different choices, not the least of which is the Librarian for access to the Divination and Telepathy trees (both excellent) or one of the named characters for something a little fancier- Mephiston is well-known as a beast in a fight and remains that way, but Astaroth can bring a Death Company brigade to an aggressive list that already has enough scoring. Speaking of which, the Troop slot is similarly blessed; ASM make excellent aggressive (with jump packs or Flamerbacks) and defensive (with LasPlas) units, while Sniper Scouts are a cheaper option in the latter regard. Death Company and Death Company Dreadnoughts can also be brought, giving and absolute plethora of options.
Looking to the extra slots you can get, Assault Terminators and Furioso Dreadnoughts stand out in the EL section, though Sternguard and Sang Guard are also there. Land Speeders are the natural FA choice, as Bikes are better taken from the vanilla Marine book; Baal Predators might have a place, though they get a bit worse in the new edition. Stormravens, Dreadnoughts, and Predators as well as Devastators mean a wide variety of fire support options; the SR can also be used as a transport, but it's doubtful good advantage can be taken of this given the points cost of doing so.
Chaos Daemons
Unfortunately, though Chaos Daemons get drastically better in the new edition, as allies they don't have a ton of benefits. You do have access to some very good HQs- the Tzeentch Herald on chariot (no, not that kind of chariot) is reasonably cheap and quite tough, bringing a pseudo-Meltagun and a great huge pile of wounds. Fateweaver and Bloodthirsters are both very strong threats, and 'Weaver is one of the few units to share his abilities with allied units. Unfortunately most other forces regard them as Desperate Allies, so if you're getting any use out of him you're probably also screwing yourself over at the same time. In the Troops section are choices are pretty sparse, so it's unlikely you can find anything impressive to do there. Other slots are also generally unimpressive, getting you some nice assault units but nothing that utterly outclasses what you could otherwise have accessed from another army.
Chaos Space Marines
Well, you've got a book with poor HQs, poor Troops, bad choices in most slots and that does the same basic things as the many, many other Marine books. I would want this why, exactly? There are several specific abilities that you might want to get from a CSM ally- an Icon for accurate DSing and Lash for an army that actually has the firepower to back it up being the two main ones- but for the most part you will give CSM a pass unless you are doing something fluffy.
Dark Angels
Terminators. Do you need some Terminators? DA is eager to help. Since Belial is a pretty reasonable number of points those Terminators will even be scoring, and you can bring TH/SS and Cyclone Missiles to the party all day long. The fact that those Terminators are also resilient scoring units is just a double bonus. You have the option of bringing Ravenwing or other support units, but most armies won't see much point to doing so, unless you have a major need for a mobile beacon. They do get cheaper Typhoon Speeders, for what it's worth.
Dark Eldar
DE are very, very fast and can be used as a contingent to move alongside with an enemy that is already crossing the board or as a detachment to do so on their own. Unfortunately, like many of the other "evil" factions they are stuck with being Desperate Allies to many people, which means you can't hope to use them to claim distant objectives of deny the enemy their own with a quick grab at the end. However, you can bring some very heavy firepower with a DE contingent- Dark Lances, Blasters, Haywire, and more are all accessible for a minimal point cost, and I think this is your best answer. DE are also one of the ways you can bring in an allied flyer- they can't claim to have the best such unit, but the Razorwing is a perfectly fine vehicle for what it does.
Eldar
Eldar are still the kings of psychic defense, and given their propensity for allying (or, more accurately, for sitting on the backlines and manipulating their allies) they can be very useful in that regard. A cheap Farseer with Runes of Warding and one psychic power (which will almost inevitably be a Divination BRB power) is a great fit into almost any army that wants better psychic defense. (An Autarch can also bring a reserve bonus, but with the improvement to reserves this is less necessary.) Dire Avengers fill the requisite troop choice for a minimal amount of points, but a better option will often be Rangers- they may not be quite as bargain-basement in price, but the ability to allocate AP1 wounds shouldn't be underestimated. The upgrade to Pathfinders is debatable, but if you take it, it will mostly be for the cover save bonus rather than the additional AP1 wounds.
In other slots D-Cannons have come into their own and are a very viable choice, albeit not a cheap one. Wraithlords seem like a strong addition, but since the only army that could really take advantage of cheap MCs isn't allowed to ally, they're largely pointless. Fire Dragons can be a solution to heavy vehicles and Warp Spiders to lighter ones, though in both cases you have to consider whether their total cost beats out that of similar options from other books. By and large, Eldar as an ally will be something you take for the HQ and HQ alone.
Grey Knights
With extremely cheap Troops (via Coteaz), HQs (with other Inquisitors) and a variety of solutions available in both melee and shooting, GK are an interesting choice. In terms of HQs it's hard to justify one of the GK choices over the infinitely cheaper Inquisitorial ones, especially when the latter bring so much utility with the potential for BRB powers; this is largely because GK are not Battle Brothers with anyone and thus can't use a Librarian's buffs to benefit the main army. However, their Troops can be excellent specialists (like the tri-Melta Henchmen squad) and generalists (like Strike Squads or Terminators), giving an ally a variety of potential options in that regard.
Outside of the necessities GK has a number of efficient units that have gained great infamy- the Psyfleman for tank-hunting (a job at which it still excels, by the way; it only lost out on survivability), Purifiers and Paladins as multirole units, Interceptors for disruption, Purgators for infantry-clearing, etc. However, any army that looks to bring GK as an ally will have to keep in mind that doing so is likely to be a quite expensive proposition, especially if any significant number of models are taken.
Imperial Guard
Guardsmen are basically the skanks of 40K, willing to go with anyone who happens to come across them. With many cheap (if not the cheapest) units available and excellent Troops, IG will be a major contender for allies in a variety of armies; their HQs can either bring some utility (from a Lord Commissar or Primaris Psyker) or general firepower (from a Command Squad); their troops are going to most commonly be an Infantry Platoon, as Veterans, which a bit cheaper, effectively can only fill roles that most armies can do better. (Some, however, can make good use of the disposable melta and may go with them.) Platoons generally can do an excellent job of holding a backfield objective while putting out reasonable firepower, especially with the addition of a Commissar or other character to bolster their morale. The required Platoon Command can be taken with an Autocannon, Sniper Rifles, Melta, or Flamers, depending on your needs.
Outside of that, the Psyker Battle Squad opens up some the possibility of Ld-based flamer tricks again (via GK or Necrons) as well as plenty of other goofy nonsense, not the least of which being the Telepathy discipline. Stormtroopers can provide a pinpoint firepower drop, while Vendettas have only gotten better and can ferry troops while deploying extensive firepower. Russes and artillery pieces can bring some punishing (and, in the case of Russes, survivable) firepower at range for armies that lack it- the Manticore is still a standout here.
Necrons
If you're looking to bring flyers or torrent shooting, Necrons are your go-to this edition; they can bring more of either than any other army in the game right now. Your picks for HQ are... well, you don't really have one; you WILL bring an Overlord and probably mount him on a Barge with Warscythe, but how you set up your Court and Troops has quite a few more options. Immortals or Warriors, Arks or Scythes, Tesla or Gauss, Storm or Destruction- all of them have a number of uses and can have arguments made for them.
In your optional slots your choices are somewhat less diverse due to the nature of Necron list-building, but you still have some very good stuff- basically everything in the FA and HS slots (save the Monolith and Destroyer) are pretty good and can find their way into some kind of army. From harassment (Tomb Blades) and countercharge (Wraiths) to anti-tank (Heavy Destroyer, Scarab) and anti-infantry (Annihilation Barge, Doomsday Ark) and even more niche jobs (Doom Scythe, Deathmarks) you have tons of excellent options to fill almost any kind of job you could want. The only real limiting factor on Necrons is their poor relations with most races- Desperate Allies are the norm, although you get Allies of Convenience with a few select armies.
Orks
Other than perhaps IG, no one brings weight of bodies like Orks do, and for many of the other, more elite forces this can be a real benefit. In previous editions the Big Mek might have been your prime choice, but with vehicles getting easier cover there isn't a strong reason to bring him along- a Warboss, especially one on a bike, is probably a better option. Your troops almost certainly want to be Shoota Boyz- with the addition of Overwatch, the "charge the Orks to deny them S4" strategy is much, much less attractive and they can provide a reasonably-cheap scoring presence. Mobs of Boyz are easier to kill now that cover is a bit trickier, but T4 is still good compensation and Fearless through the majority of the squad's life helps as well. Grots as a super-cheap scoring unit are a possibility, but really not many armies are going to need that- if you're bringing Orks, you're doing it because you wanted melee defense.
Most of the other Ork choices are not great, though they can be usable. Nobz and Nob Bikerz (both potentially troops as well) can give you a strong aggressive element, and Lootas are an interesting solution to the dilemma of flyers (though they are very vulnerable to return fire.) Big Gunz are cheap fire support but not many armies lack for such an option; Buggies are less attractive now that vehicle-blocking and squadrons work very differently. Most of the other units quite simply have no place in a real army and shouldn't even be discarded.
Sisters of Battle
Sisters... really don't bring a lot to another army because, to be honest, they don't bring much to their own army. That isn't to say they're awful, it's just that you will never be impressed with what they offer you because someone else can do the same job with better side benefits. Uriah is as much a no-brainer as ever, since he brings along the excellent Conclave, although there is an argument to be made for the utility of Celestine. You have no options for troops- you bring a Battle Sister squad. If you're doing this anyways you might as well take some Dominions, Retributors, or an Exorcist, but none of them are gonna be so good as to blow you away.
Space Marines
Although their HQs aren't exactly cheap, Marines stick to their core strength by being able to provide a number of unique options. A Librarian, of course, can give mobility (with Gate), debuffs (with Null Zone- Daemons are relevant again and cover sucks), or any of a wide variety of BRB powers. Beyond the obvious, however, a Biker Captain can bring some very mobile troops along with him and a Master of the Forge can give you a crazy number of Dreadnoughts on the field. Tactical Squads make for good, survivable scoring, no matter what the internet may say (remember, Combat Tactics and heavy weapons are both good things) and Scouts are a reasonable choice as well.
From there you can select almost any kind of firepower (TFC, Predator, Speeder, Attack Bikes, Sternguard, Dread) that you could want as well as the sheer melee prowess of TH/SS; Marines really are jacks-of-all-trades, and none more than the vanilla codex. The fact that they have Battle Brother status with perhaps more armies than anyone else (and certainly more varied ones) doesn't hurt, either.
Space Wolves
Space Wolves aren't a better SM, they're a better Eldar. What? Yes, really. Did you need psychic defense? Well, they've still got it. Want access to Divination for rerolls? They have that, also, and their starting powers are pretty snazzy to boot. You don't really have a good reason to take anything other than a Rune Priest, although maybe a Battle Leader or something could be justified. Grey Hunters, of course, are your troops; a backfield role (LasPlas Razor) is possible, but really doesn't make good use of them- generally they'll be more attractive when you're pushing forward.
Other slots will be the usual characters. Wolf Scouts are good disruption and Melta, Thunderwolves can provide pretty frightening countercharge, Long Fangs and Dreads bring fire support on the cheap. All of these are nice, but not particular deal-breakers; you'll really only be looking at them to fill out a list when you're taking SW for the HQ choice.
Tau
So you have to take an HQ; that's kinda a downer, all things considered. Occasionally this will be nice because a Positional Relay, while pretty garbage in Tau, can serve some interesting uses in other armies in controlling the flow of reserves. You also have to take some other guys, but Fire Warriors are less trashy now (and Kroot are reasonable, if not great.) You won't be too sad about your mandatory picks, but neither will you be very excited.
The good stuff for Tau comes in the other slots- Broadsides and Crisis, mostly. If you need some high-strength, low-AP firepower, Tau can do that for and on a reasonable budget to boot. Piranhas, like many similar vehicles, loose a fair bit of "oomph" in this edition but can still serve uses at times.
Now, it's also worth reiterating how the different types of allies function. Battle Brothers is the big one- such units are, in most ways, just a regular part of your codex, although they won't benefit from codex-specific abilities and can't get into transports; usually these are inter-Imperial alliances, although it's worth noting that even such forces don't always fall under this category.
Allies of Convenience can't be affected by your psychic powers and such, but otherwise count normally. They won't incur any real problems and can generally be integrated freely into your army, although it is worth remembering their limitations- they are still technically enemy units, which will matter for certain abilities (like Imotekh) and will trigger some psychic defenses (like Shadow in the Warp, Runes of Warding, and Runic Staves.) Desperate Allies are the real hard choice- they are non-scoring and non-denial (negating any value of their troops, generally) and if any of your other models are near them, they have a 1/6 chance of being too suspicious of them thar furners to do anything else. (Racism is an important part of 40K's setting.)
So when you're thinking about what roles you need to fill in your army, keep your allies close in mind- many codices can benefit extensively from mixing in someone else's specialties (be they from HQ, Troops, or other slots) to cover gaps in their own. Allies are a huge part of list-building in 6E and taking proper advantage of them- but not overspending- is a critical thing to learn.
