Interesting topic. I would say - definitely Zerg. Sure they not as scary as Xenomorphs or Tyranids, but those you can at least fight somehow.
While zerg... well, they can basically asimilate EVERYTHING that's at least a bit biological (even other aliens) and then: control it/use it genomes to evolve/hatch Broodlings/use as a living bombs/or just mindless weapons. They can be small, big, colossal, fast, heavy, light, fly, move underground, attack via distance, brutal force or simple infection. They also come in heeeeeavy numbers, 'cause the more life is on the planet, the more biological resources they have. Oh, and they can always bury if things are going bad for them.
The only way to effectively fight them is to blow the planet to freakin' pieces. And even then they could've escape 'cause can also travel through space.
So yeah, Zerg aren't really that scary, but to fight them... you're pretty much screwed right of the bat :]