I'm suprised the Necrons haven't been mentioned.
They seem pretty cool with the whole Eygptian angle, but their fluff is incredibly under-developed.
Also if you are talking of overpowered, drop Space Marines from the conversation and vote for the Necrons. When they wake up, the fluff basically says ... "Your all screwed!". Which is incredibly one dimensional.
I'm also suprised people hate the Eldar, they have for my mind, one of the best pieces of fluff ever written, 'The Fall'.
Basically take a long look at the Dark Eldar, thats what the Eldar had become by the time of The Fall, how awesome is that?
Also 'regular' Eldar may be a little space hippish, but the fact they are f*cked and are still able to run rings around most enemies tells you an awful lot about them and their powers. Also viewing the Imperium as monkeys is awesome, just like:
So like I said, until their fluff is developed more, for me, it's gotta be Necrons.