On the tabletop, I have to go with Space Marines. More specifically, Blood Angels. Even more specific, convert the entire army to Death Company. I have 4000 points of dudes who have a 50% chance of ignoring any injury sustained. Yes, they cost more than other Marines, but the benefit of having a Chaplain commander who can have 20+ guys in his squad who can all wield power weapons is way too cool. Also, I've gone insane in large games and taken Eldar units as allies, just for kicks. A Death Company force with a few squads of Eldar is too much fun and always has opponents saying "Awesome! Wait. What the hell?"
In DoW it was the Eldar or Necrons for me. Sorry, but the Marines got boring after a while.
DoW2 - Eldar or Tyranids. Not really sure why in that game.
Suggestion for more video games in the franchise - Use the tabletop points system instead of pop cap. In the case of DoW2, have a set point cost you can bring in per mission, instead of a choice between 4 kind of lame heroes. It would be kind of like MechCommander 2, which would be AWESOME!