The Borg from Star Trek: Armada
(Yes, it's an old game, but it still counts.) That game lets you play as human, klingon, romulan, or borg. The other 3 races just try to blow each other up, but as the borg, the other races become another resource. hehe. I love sucking the crew out of their ships with the Assimilator borg ship, taking over their ship, and then using their own guys & ships against them. 8D I still play that game every once in a while.
Note: Armada 2 is nowhere near as good as the 1st one. Try as I might, I just couldn't enjoy it.
The Borg are just plain cool. If they understood psychology and how to manipulate the races with something other than force, they'd be invincible, but that's another story.