IMHO:
Alien: Good
Aliens: Better
Alien3: Not as good, but not a waste of time, either.
Alien Resurrection: Top 5 worst movies I've ever seen. Killed the only character I liked, first. Best part of the movie, wasn't even in the movie. Do you remember the scene where all of the scientists were stuck up on the walls? My buddy, having stomached about all he could, stood up in the middle of the theater and started singing "Always look on the Bright Side of Life". The audience (all 10 or so of us) died laughing.
Well, maybe you needed to be there. But, geez, that was a baaaaaad movie.
To me, the Alien movies stopped at #3, and I barely liked #3 but it was interesting.
Dumbest thing in Sci-Fi... an alien, an armored alien at that, gets sucked out into space, through a BULLET HOLE. Never mind that you could simply plug up that hole with the palm of your hand.. noooo, they spaghetti-fied poor sad Mr Alien into space...