First impression: leaning toward the dragons. They have on their side numbers, individual size/strength, intelligence, flight, tough hide.
Humans with swords and such? Might as well fight with barbecue sauce.
I see no reason to think the Z-virus would effect the dragons.
Dreamcatcher aliens: gunships/dragons, same result.
The Mist creatures...too many unknows about them. The big one seemed slow as molasses; and I see the little ones doing about as well against the dragons as the PB flyers.
The Preds, I think, would last the longest against them. To really beat the dragons, I think the Preds would have to devise a global strategy, and I don't think they have the numbers to do that, even with mini-nukes. The Preds could maybe get a pyrrhic win.
The Xenos...depends IF the facehuggers could infect enough dragon eggs/babies; and even if that did begin to occur, I could see the dragons catching on real fast and nipping it in the bud, fire-saturating caves and such.
Hell, the dragons may even be smart enough to let the other factions fight each other, to make their work easier in the long run.