Notes:

Honestly, this ones a toughy.

The Necromorphs and the Flood would find themselves in a natural stalemate as they continuously convert each other; unless the Flood can get a technological advantage to help them in the fight.

The Demons might come out on top, as they burn to ash upon death so they themselves won't be subject to either Necromorph or Flood infection; at least not effectively. However, Marker induced suicide and the Flood "borrowing" any technology they come across could tip this into another stale mate.

P.S: I've spent a while trying to track down the original source for this and this is the most likely one so far. If anyone has access to the actual proper source, let me know and I'll change it ASAP.