Its a tie between Eldar and Space Marines.

Eldar- Speed, stealthed bases, teliportation, webway gates, commander + seer council, avatar, wraithlords, etc.

Space Marines- Toughness, versatility, heavy bolter tactical squads, drop pods, terminators(especially the hammers), orbital bombardment, etc.

SM are by far the best race statistically. The reason being, is that they can drop an entire force anywhere on the map that they can see, and that force is the toughest force in the game. Unless you drop the force in the middle of a turret field or an enemy army, it just won't die. By the time anything does get destroyed, another drop pod will be ready. SM, being tough all over, can easily shake off rushes. Anyone who has played as all the armies would agree, SM are the most versatile race in the game.

Eldar are probably the most specialized and hardest to use. IG can be hard to win with as well, but eldar need much more micro managing. However, if the micro managing is done correctly, and you can get to where you have a support portal, the eldar just simply win. In this order(or simultaneously): Make 2 dark reaper squads, make avatar, make 3 warp spiders, make seer council, make farseer, make 2 fire dragons, and make 10 wraithlords(if you have enough webway gates when you create the avatar, your vehicle cap jumps up to 30 and your squad cap goes up to 24). This force is unstoppable when properly used.
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Orks Orks Orks Orks are the best i mean 2 flash gitz squads + 2 nob squads + Gorgutz = lots of death and waves of waves of them is awesome
second are the eldar doing the whole webway portal bring army through into there base is awesome
Tau because fire warriors just mow down anyone with ease and the commander is his own tank missles flamethrower and awesome chain gun
And last come THE TYRANIDS as there probably the only race that can outnumber the orks and there way strong there only problem is that synapse ****
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obviously my current favorite are the necrons as they are unstoppable when they get a big enough force.

however i also like eldar as they have lots of specialised troops and the relocation matrix is kickass when you relocate a load of d-cannons into an enemy base:D:D:D
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My ranking of races:
1: Space Marines (also first race I played....no not Dow but chaos gate.)

2:Imperial Guard reasoning: I love their artillery and the fact that their crappy morale gives humours sayings like..."We are the Imperial Guard and its time we start acting like it..run away".

3:Chaos: Why because nothing is more fun than sending an army of cultists to there deaths...well grot do give them run for their money.

4ark Eldar: Well I like them now after My friend and I moded them.

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My favorites are:
Eldar: I can be really annoying with them.
Tau: Gotta love superior firepower, even though not being able to build turrets sucks a bit.
Although some of people here seem to think that Eldar aren't any good, you just have to use everyone's special abilities and make sure you have the right guys shooting what they are good at, they are a bit of a micromanagement force. Tau just have really good weaponary, I love harbinger drones, they can be really fun to use.
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