I pick the Xenomorphs. Not only are they usually plentiful in number, quicker than you, stronger than you, and most likely tougher than you, as mentioned by another user, their modus operandi in the precense of humans ranges from skull-fucking them and letting their babies eat their way out, or simply eating the humans themselves. Oh yes, and we can't forget about the acid blood that violently disintigrates everything that isn't Xenomorph skin.
At least Predators have some honor. They think more on our level. They'd be like a ridiculously tough squad of human enemies, only faster, stronger, and uglier. We'd simply have to outsmart them and/or use some of our more advanced weaponry, because in this day and age, there's some fantastic firepower. They'd be a pain in the ass by all accounts, but Xenomorphs win by sheer number and excessive brutality (though Predators are pretty damn gruesome themselves; they do like to skin people).
Borg? No. If I recall, the Borg are mainly cyborg Federation personel. Being that most Federation personel tend come off as, for lack of a better word, pansies, I wouldn't be too worried about them. In a day and age where it would be possible for the Borg to exist, we'd have weaponry advanced enough to decimate them. Hell, we've got the weaponry now (nukes, anyone?).
I consider the Zerg Xenomorphs who don't melt you when they die. Very very close second to the Xenomorphs. The Zerg are just plain evil.
I place the Flood in almost the same category I place the Zerg. They're heavy in numbers, they take over the bodies of dead people, and did I mention that they're fucking heavy in numbers? It takes a weapon of intergalactic proportions (one that when fired probably makes all of the nukes in the world going off at once look like a sneeze) to end them. But why do I pick Xenomorphs over these bastards? Simple. The Flood will not rape your face and have their babies eat you from the inside out.