Why anything but the Imperium? Man is always destined to win, regardless of whatever other side-races are introduced. They are the only ones who would even have the capacity to rule a galaxy.
The Forces of Chaos will just end up raping and destroying everything until there's nothing left, so they get bored, and either kill each other or, better yet AND more likely, turn against their gods for shits and giggles.
The Orks would do much the same, but probably be fine with it. However, they have no physical gods to turn against, so I suppose they'll just leave this galaxy for another.
The Eldar are pretentious, pompous, sanctimonious little space-elves who already had their chance at galactic domination and screwed up royally. They should all curl up and die or join the Imperium... oh, wait. No Xenos, forgot. They should just curl up and die.
The Necrons will release their eldritch Lovecraft gods over-and-over again, like the cheating, game-breaking bastards they are, until they kill everything and tehy just go to sleep. Not a very good form of galactic rule.
The Tau are just upstarts with cool technology and none of the endless power of the Imperium. Honestly, if the Imperium didn't already have Chaos, Orks, Tyranids and everything else breathing down it's neck, do you think the Tau would have kasted as long as they had? Plus, they're Communists. They can't rule a galaxy.
The Tyranids will just eat everything and spit it back up before leaving.
Thus leaving only man. Man will win, as he always does.