Answer Probably Necrons.
As it stands, no, the rules are severely outdated, though there are rumors of necrons being updated sometime before the end of 2011. If that is the case, then the Necrons will be a viable army to play again.
Start with half your necrons in reserve deep strike monilths into enemy lines and let the the necrons pour out
Get a night bringer and pariahs.
Necrons
If the blood angels have a couple of squads of Death Company with jump packs, Astorath, and two furioso dreadnoughts with drop pods, you can grind the necrons in assault. But if the Blood angels don't make it, the necrons will flay them in shooting. If a C'tan is there, forget assault, Sons of Sanguinis.
All are good at diffrent things Tau: Great for long range but not much else Tyranids: Great for swarming in close and personal and ripping them up Chaos: Good all round troops well balanced for beginners Necrons: Think you kill them they get back up! damn things don't die! They have great leadership as are robots Overall id say necrons or tyranids. i personally prefer necrons but that is just me.
Theoretically, any faction can team up with another, as long as you have a good enough reason. Technically, Necrons and Chaos are enemies, so they would never 'team up', per se, but one could create a feasible story in which they both work to exterminate the Imperium. They wouldn't be working 'together' (because Necrons would kill any living thing--including Chaos Marines--on sight) but they would have the same goal.
They Are The Second Best 40k Army Below Tau And Just Above Necrons
Teleport the necronlord close to the warp portal change into the night bringer and unleash all hell.
AnswerImperial Guard Eldar and Tau Chaos, Necrons, Orks, Tyranids and Dark Eldar are considered the "bad guys".
No. The only armies in the game are Chaos or Space Marines. Being a necron player, I too wish they were in Warhammer 40k: Squad command.