I'd say my favorite is Aliens. I really enjoyed the development of Ripley's character, and the marines were awesome [especially the "Game over, man!" one
Sorry, I've only seen it once
]! I've gotta see it again, the rewatchability is insane.
Then again, you can't beat the intensity of A L I E N (so intense that I fitted my username to match the title's sheer intensity
). I swear when Ripley had to run through the ship alone toward the end I nearly fainted- the pacing was absolutely killer [er, no pun intended] and the music while subtle, gave me the chills and sold the creepy atmosphere perfectly. Also, the scene where Dallas(?) is in the air ducts was cringe-inducingly scary.