Alien is the best. Of course. What praise can I lavish upon it that hasn't already been expressed in this thread.
I rank Alien 3 above Aliens (and it's not close). I appreciate Aliens, I think Aliens is a super watchable, fun movie, but the edgy, nihilistic 90's kid in me just loves Alien 3 as a drama about people who are trying to accept/have accepted their mortality and unimportance in a bleak, uncaring universe. Plus it's shot/designed like Warhammer 40K or something.
Aliens to me...It's like all I can do is think of it as a movie designed to be edited down to 70 minutes to watch on TBS on a Sunday afternoon while cooking or something. A classic, to be sure, but not one that requires a dark room and constant attention.
I might watch Resurrection if I've been drinking and want to chuckle at corny Whedon one-liners while watching a cavalcade of character actors (Michael Wincott! Ron Perlman!
Brad Dourif! Dan motherfucking Hedaya!) get fragged to the dutch-angle infused stylings of the madman Jeunet. Also 90's Winona Ryder with a pixie cut.
I watched Prometheus once on its release day. I don't think I hate it -- I actually remember walking out of the theater kind of liking it -- but I don't want to watch it again. Maybe this week if I go see Covenant.
AvP/2 -- never saw, never will, don't give a shit