It all depends on who the nids fight first.
If they fight the orks - Nids win. they get toooo massive to stop, just due to the VOLUME of organic material they would garner.
If they the Necrons, its a toss-up between Orks, imperium, and Chaos; cause' Crons don't produce any digestible material. Even in the game, the wounded (that don't stand back up) get teleported back to a crypt for more strict repairs. HOWEVER; when eaten, they arn't getting rebuilt; and no sigle force can stop the Nids. The metal isn't indistructable, it just grows. If disollved? bye-bye metal. The problem is that the Nids probably can't get anything from eating the Crons. I know it says in the books that Nids avoid Crypt worlds, but Nids ARE life in its most primal form, something the necrons are programmed to stamp out. With Crons gone and Nids not growing back hives from removing them, the shear losses will feed the deamons of chaos to a size unimaginable, the Orks will have more focused enemies to fight, and the Empyrium will keep on chuging along do to its size.
With 2 of the 3 splinter hives coming in surrounding the Tau empire, and the one that is halted by the marines is right on top of thier 3rd expantion. The other two: they are gonna blow right through the Tau empire before they fight Majority Crons or Orks. in other words - they gone! Both groups of eldar are hosed, since the are dying faster then they are reproducing; end of story; I don't care that the dark eldar have a webway city. They are doomed by Chaos invasions. Harlequins have a shot @ living forever in the Webway, but as seen w/ the experimental codex, they don't make an army themselves. All that is left is Humans, Chaos humans, and Orks. Orks make up the majority of life in the Milky way right now, Humans and Nids coming in behind. Chaos will keep fighting until the Eldar + Emporer are gone, then nothing will hold open the Eye of terror as too little will be feeding the deamons. If they hold out, they can win, but they won't hold out. The empire? only runs due to its volume. If wars begin to stop, they will have too much time to think about themselvs, and the entire thing will colapse unless the 'emprer's long lost sun' or some such bull can pull it together.
I'm banking on the Nids myself, Orks in the second slot, Humans third (until they collapse in on themselves being the last man standing)