Hmmm the simple stat to look at is how they reproduce. The flood are an assimilate species taking over hosts to swell their numbers similar to the xenomorphs except they infect their host mutating adding the hosts traits to the new offspring. Since the floods hosts are unstable and some even explode when pierced it's unlikely the facehugger would be able to use an flood infected lifeform as a host. So I'd say flood in that fight

Heh.. I see.. but the other one now.. Zerg or the flood?

Well, given that both the Xenomorphs from Aliens and the Zerg have only required nukes or glassing an entire planet, while the Flood made a whole civilization commit both suicide and possible genocide...

I'm gonna lean toward the Flood until someone makes a rational argument about how the other two could win. =P Though I'm biased toward the Zerg, so I always favor them in a fight, even if they don't win. X3

The only problem I see with the flood is they out eat their food. That's the only problem. They will have problems fighting the zerg tho. Which they are basically the same thing in most aspects.. but then again I have been wrong before

Fair point. It's possible that both species would push each other to their limits, forcing evolution and adaptation at a remarkable rate. The only outcomes are one overwhelming the other, or both merging into something far worse.

Makes me think of a stinger(?) from a movie poster: "Whoever wins, we lose."

Or the thing.. that was a Interesting movie