Aliens.
I respect what Ridley Scott was doing with Alien and it's a very good film, but there's a certain pomposity to Scott's films that, IMO, prevent them from ever being real classics (Bladerunner may be the exception).
Alien 3 was the weakest to my mind, the special effects were... not good. But it was still pretty decent. David Fincher is kind of like Ridley Scott in some ways, he's a good director hindered to some extent by some vague sort of self-importance (although I really loved Fight Club).
And I have a definite soft spot for Alien: Resurrection. Jean-Pierre Jeunet is probably my favourite director of all time (Delicatessen, City of Lost Children, Amelie, brilliant films). I thought Resurrection was really very interesting indeed. But it wasn't as good as Aliens.
As for AvP... I refuse to even consider them.