If you really want to practice for a tournament, the most important thing is playing games. And if you want to play practice games, the most important thing is playing a good opponent with a good list. While not everyone will be able to get highly-skilled opponents to play multiple iterations of a matchup with them, it's still something to aim for.
More importantly, it's critical, if you are aiming to have a strongly competitive list, to know how your list lines up against various other armies out there. WH40K isn't Magic- you have no sideboard and matches are only one round; because of this, you can't afford to have a truly bad matchup against any of the major armies. If you find yourself unable to beat a significant fraction of the field due to innate weaknesses or strengths in your respective armies, you may have to alter or discard your list, because that isn't really viable for playing competitively. Betting on rock, paper, scissors to go your way is a terrible risk to take; if you build your army well, you can manage much better percentages for much less sacrifice.
This, then, is an attempt to compile the armies I use (when possible) to playtest against. It's largely expected that you'll have to proxy to do so, and keep in mind that these are NOT the "best" possible lists- these lists are designed to be representative of the way these particular armies function while still being relatively simple and easy to play (to make things less stressful on your playtest partner.) You certainly should be giving your playtest opponent all possible advice as to how to win, as when facing a "real" army you will likely be seeing a much more experienced and knowledgeable general at the helm.
(I am listing 1750 versions here for simplicity's sake; you can expand them up or down as necessary in most cases.)
MSU MEQ Shooty
A-derp. This is the list everyone whines and bitches about (right now) and it's the first on our little checklist. SM, SW, and BA versions are certainly not identical, but I can't really blame you if you only test against one of them, as they are similar enough to make generalizations from. The list scales fairly well in either direction, so it shouldn't be problematic.
1750 Space WolvesImperial Guard
1 Wolf Priest (Lightning, Stormcaller)
1 Rifleman
1 Rifleman
10 GH (2 Melta, Rhino)
9 GH (Melta, Rhino)
5 GH (Melta) (LasPlas Razor)
5 GH (Melta) (LasPlas Razor)
5 LF (4 Missile) (LasPlas Razor)
5 LF (4 Missile) (LasPlas Razor)
5 LF (4 Missile) (LasPlas Razor)
Bunch of slightly-different vehicle hulls backed by long-range shooting and Melta. MSU like the Marine list above, but its units fold more easily once you disembark them. All Chimeras are Multilaser/Heavy Flamer setup.
1750 Imperial GuardMechanized DE
1 CCS (2 Melta, 2 Flamer)
5 PBS (Chimera)
5 PBS (Chimera)
10 Vets (3 Melta)
10 Vets (3 Melta)
Infantry Platoon
-PCS (4 Flamer) (Chimera)
-Infantry Squad (Commissar w/PW)
-Infantry Squad (PW)
-Infantry Squad (PW)
-HWS (3 AC) (Chimera)
1 Vendetta
1 Vendetta
1 Vendetta
1 Hydra
1 Hydra
1 Manticore
While the above lists test your ability to handle large numbers of resilient shooting units, this list is more about fighting fast, mobile units with high threat output. Even less resilient than the IG list, what it lacks in toughness is makes up in ability to murder infantry models of all stripes. All Venoms are assumed to have the extra Splinter Cannon bolted on.
1750 Dark EldarGrey Knights MSU
1 Haemo
3 Trueborn (2 Blasters) (Venom)
3 Trueborn (2 Blasters) (Venom)
3 Trueborn (2 Blasters) (Venom)
8 Wyches (Haywire, Shardnet) (Raider w/Shock Prow)
8 Wyches (Hayrire, Shardnet) (Raider w/Shock Prow)
5 Warriors (Blaster) (Venom)
5 Warriors (Blaster) (Venom)
3 Wracks (Venom)
6 Reavers (2 Heat Lance)
6 Reavers (2 Heat Lance)
1 Ravager
1 Ravager
1 Ravager
Differentiated from the above Marine versions because it functions rather differently in major ways; it uses different guns, deploys and moves differently, etc.
1750 GKJumpers
1 Inquisitor (Terminator, Psycannon)
3 Acolytes (Meltaguns) (Chimera)
5 Purifiers (2 Psycannon) (Psyback)
5 Purifiers (2 Psycannon) (Psyback)
10 GKSS (2 Psycannon, MC Daemonhammer, Psybolt) (Rhino)
10 GKSS (2 Psycannon, MC Daemonhammer, Psybolt) (Rhino)
5 GKSS (Psycannon) (Psyback)
5 GKSS (Psycannon) (Psyback)
1 Psyfleman
1 Psyfleman
This is the premiere Deep Strike army in the game right now- Drop Pods and Daemons give it their best, but for the most part Blood Angels is just tougher, hits harder, and has more options. Note that it's not a pure drop army but can start both on the table or in reserve (albeit at a slight disadvantage on the latter compared to a pure list) as the situation warrants. While Jumpers may not rank quite as high as some other lists due to a couple bad matchups these days, they are still very dangerous and even in their "bad" fights they can easily pull out a win if the enemy isn't careful.
1750 Blood AngelsTerminator Wing
1 Librarian (Jump Pack; Shield, Lance)
5 Honor Guard (3 Melta, 2 Flamer)
2 Priest (1 Jump Pack + Power Weapon)
10 ASM (2 Melta, Infernus, Fist)
10 ASM (2 Melta, Infernus, Fist)
10 ASM (2 Melta, Infernus, Fist)
10 ASM (2 Melta, Infernus, Fist)
6 Devs (4 Missile)
6 Devs (4 Missile)
This army, like Jumpers, asks you a really simple question: how many mans can you kill? Although lacking the ability to arrive as accurately, it still brings good firepower the turn it shows up and has better long-ranged options as well as being an absolute beast in combat.
1750 Dark AngelsDual Raider
Belial (LCs)
5 Deathwing Terminators (Cyclone + Chainfist, 4 TH/SS, Apothecary)
5 Deathwing Terminators (Cyclone + Chainfist, 2 TH/SS)
5 Deathwing Terminators (Cyclone + Chainfist, 2 TH/SS)
5 Deathwing Terminators (Cyclone + Chainfist, 2 TH/SS)
5 Deathwing Terminators (Cyclone + Chainfist, 2 TH/SS)
5 Deathwing Terminators (Cyclone + Chainfist, 2 TH/SS)
1 Land Speeder (Typhoon/Multimelta)
1 Land Speeder (Typhoon/Multimelta)
Some lists just have a couple big, stupid threats that they throw at you and hope will punch you to death; while this may not be exactly reliable, it can be pretty devastating if you are unprepared or if luck turns against. Beating them requires a sound ability to judge distances and use blocking tactics will be critical to beating these armies. Other varieties, of course, are possible (Stormraven, Battlewagon, foot rocks, etc) but this is probably one of the more flexible and tricky ones, relatively speaking, so it's your best bet for testing against.
1750 Space MarinesTyranids
1 Librarian (Terminator; Null Zone, Gate of Infinity)
5 Assault Terminators (TH/SS; Land Raider Crusader w/Multimelta, Extra Armor)
5 Assault Terminators (TH/SS)
10 Tactical Marines (Missile, Melta; LasPlas Razor)
10 Tactical Marines (Missile, Melta; LasPlas Razor)
1 Land Raider Crusader (Multimelta, Extra Armor)
1 Predator (Heavy Bolters)
1 Predator (Heavy Bolters)
Wait, aren't Tyranids bad? Not exactly, no. Like Jumpers, they have several weak matchups, but for armies other than those they can still be a very tough competitor, especially as you sink below the 2K mark and their anti-tank becomes less problematic. Tyranids are a good test of your ability to handle masses of infantry targets with cover and good melee ability; they can easily field a swarm that most armies will have a hard time chewing through and simply overwhelm the enemy with targets while playing to the objectives. However, more so than most armies they are reliant on good terrain coverage on the table and thus can easily suffer problems depending on the board setup.
1750 TyranidsAn astute read will quickly notice that not all codices are represented, or even mentioned, in the above archetypes; this is not an accident, though nor is it either to say that such codices/armies are not viable choices in any way. These lists are for the purposes of testing, and where other lists overlapped them sufficiently (such as in the case of different MEQ MSU shooting armies from the SM, BA, and SW codices) I have combined them together. A wise general will know each of the different books and how they differ from the others, but unless one has an overwhelming abundance of free time, it's simply not feasible to playtest extensively against a dozen or more different armies.
1 Hive Tyrant (Heavy Venom Cannon, Whip/Sword, Old Adversary, Paroxysm, Leech Essence)
2 Tyrant Guard (Whips)
3 Hive Guard
3 Hive Guard
2 Hive Guard
10 Termagants
10 Termagants
10 Termagants
1 Tervigon (Cluster, Catalyst, Adrenal, Toxin)
1 Tervigon (Cluster, Catalyst, Adrenal, Toxin)
1 Tervigon (Cluster, Catalyst, Adrenal, Toxin)
1 Tyrannofex (Rupture, Cluster, Dessicator)
Additionally, there are a number of other armies that you should understand how to beat, since at least some of them will certainly make appearances in any large tournament, even if they fall well below the threshold for what I consider "good lists." That doesn't mean they present no threat to you, especially if the TO has come up with some "wacky" (read: usually bad) missions, but all other factors being equal, they shouldn't be hard to beat.
-Dual Lash
-Green Tide/Dread Bash
-Fatecrusher (and/or other Daemon lists)
-Foot Marines/Footdar