Probably the Orks or Tyranids. They're the only one's with the numbers to pull it off.
It's been a whiles since I stopped playing, but I think according to the fluff there was a massive Tyranid fleet heading towards human space at one point. An Inquisitor managed to divert it by conducting exterminatus on a bunch of Imperial worlds and thereby wiping out the fleet's potential food supply along that route. Instead, the Nid's flew straight into and Ork Waaagh! and have been fighting each other endlessly ever since, with plenty of reinforcements arriving constantly for both sides.
Thing is, if either side ever actually won that war, then they'd be able to recoup their losses incredibly quickly (in an Ork win the blasted apart Orks spawn more Orks; in a Tyranid win there are plenty of dead Orks to eat and make new Tyranids with), and then will have an army of unholy proportions to march against Terra. All the other races are outnumbered and would soon fall, with possible exception of the Necrons in the event of an 'Nid win, since the 'Nids seem to be afraid of Necrons for some reason. Also, if the Orks win then Chaos will probably survive by retreating back into the Eye of Terror, which by being the only enemy stonghold left to the Orks apart from their own, will quickly come under assualt by perpetual waves of Waaaghs! and the two will battle again for all eternity.
