Good question
I would definitely say we would have the best chance against the Aliens . . . . followed by the Predators. I think the SST Bugs would be the most devastating.
The Aliens are basically a parasite. A highly evolved and extremely deadly parasite, but they really have no means of travelling throughout space other than by latching onto another organism. And they may be fast, but they're not immune to a good old shotgun blast to the head and besides. We could always just "take off and nuke the entire site from orbit . . . it's the only way to be sure"
The Predators may be technologically advanced, and they may be great hunters, but that doesn't make them great soldiers. Too proud and bound by honor. They would likely send a small band of warriors to take on a much larger force (as some right of passage). Then we just hammer them with everything we've got. Who said it has to be a fair fight anyway.
The bugs though would decimate us. For a start, they can hurl meteors through space (they took out a pop. of 10 million with one meteor). Once they reigned down meteor storms all over us, they use the same meteors to deliver swarms of soldier bugs and tankers. As someone already said, no fear, no honor, just a mass of thrashing limbs ready to disembowel anything that moves.
For that matter, the bugs could wipe out the Aliens and Predators as well. No contest.