Hell, I'm just going to take the thread title at face value and say I'd love to see Aliens in Star Trek, that is, the Xenomorphs.
For that matter, I'd love to see how a Predator stands up to more advanced Starfleet tech. And if we have the two face off, it could more than make up for that insipid Voyager episode where the Hirogen hunted Species 8472!
Other aliens:
-The Thing (John Carpenter/2011 prequel version)
-The Blob (y'see where I'm getting at)
-Jar Jar Binks (just so that we can launch him out a torpedo tube)
-The Time Lords (hah!)