I agree with most of the comments. AVP 1999 is good, but, in my opinion, AVP 2 was the best thing in terms of atmosphere, story and graphics. I still play the single player missions on a regular basis and love the fact that you can find some multiplayer matches every now and then.
I also think ALIEN TRILOGY is heavily underrated. I play it using DOSbox and love the atmosphere, the sound effects and ...over all, the beautiful and mysterious soundtrack. This game has, in my opinion, the best soundtrack ever made for an ALIEN game.
AVP 2010 and COLONIAL MARINES...meh