Sooo... which of the XCOMs is the bestCOMs? [I kind of want to play one now]
This is a really tricky question.
I really enjoyed Enforcer (the arcade FPS one) for what it is. I also thought Apocalypse (the third one) did a very good job in its own right. Nothing beats Terror From The Deep, though, which fixed some of the problems the first one had and extended gameplay quite a bit with stuff like two-part missions.
The rest are garbage to me, though (excluding the original).