I've never been a HUGE fan of this series, and had only seen each of them once.
For YEARS, I said the first was my favorite, but I tried watching it again recently and I had a hard time getting into it. It's good, but not as good as I remembered.
Aliens, I considered inferior, but now I like it more than the first.
Alien 3, I saw in the theater. Me and my friends laughed our ASSES OFF the whole movie. We were in high school and thought the way the British guys were yelling expletives was HILARIOUS. I tried rewatching it recently and couldn't get into it either.
Alien Resurrection, I saw when it first came out on cable when it was still a "newer" film, and I remember liking it more than the 3rd, but it has been so long now that I couldn't tell you if I'd still enjoy it.
Haven't seen any of the AvP.
Anyway, I voted for Aliens.