The original AVP game on the PC was amazing. Crazy scary (when playing the human). With incredibly dark maps and having the motion detector (movie Aliens) was incredibly creepy, especially since aliens could come from the ground, walls, ceilings, etc.
Playing the Alien was the most fun, for me personally. Going around taking out lights. Sneaking up on people while clinging to the ceiling. Great stuff.
The predator was a lot more like playing Crysis, where you are this powerful guy with all these cool gadgets.
Somehow, the AVP2 game (for me) was less scary and lost the elements i loved from the original. I would still play the original, but unlike Blizzard, they haven't released a patch for it in a long long time, and the game is really messed up on a modern system, though i think there are some 3rd party fixes.