I'd say Aliens. Of course because of those whacky space marines, but also for what I like to call "Predator Syndrome". It's something I noticed with horror movie franchises such as Predator. The reason everyone says that the first Predator movie was the best is because when you're seeing it for the very first time, it's a mystery. You don't know if it's a person with high-tech gear, a monster, an alien, you don't know what's going on. Hell, the first half of the movie only shows very seldom heat-vision shots from the Predator's viewpoint as he stalks the team that are going to take out some rebels or something. Half the movie goes by you don't don't even catch a whiff of the monster.
It was impossible for Predator 2 to match the atmosphere and suspense because all of a sudden you're dealing with a known quantity. Assuming you saw the first movie, you already know what's out there and what it's capable of. This almost demands a shift from "suspenseful horror" to more a more action-oriented story, which Predator 2 most certainly was.
It's the same with Alien and Aliens. The first time you see the first movie, EVERYTHING'S a surprise. From the facehugger to the chestbuster (I don't care who you are, the first time you see that scene you pee yourself a little

) to the vent-shaft hunt and everything else. In Aliens, all the mystery is removed, you already know what these creatures are and what they do and how deadly they are, meaning it had to go for a more action-oriented story.
Sooooooo how does this equate to me actually voting for Aliens after suggesting that the mystery and suspense of Alien make it superior? Simple...I've already seen both movies. There's no more mystery or suspense for me in Alien. I still love the movie and consider it one of my favorite sci-fi movies of all time. But all things considered, with they mystery removed the playing field becomes even. At which point I have to go with the movie with the wise-crackin' space marines. I mean come on, there's just WAY too many quoteable lines in that movie!
"This little girl surivived all on her own for 2 weeks and she didn't even have military training!"
"WELL FUCKING PUT HER IN CHARGE!"
"I say call the dropship, get back to the Sulaco, and nuke the site from orbit."
"Fuckin' A!"
And for the record, I do still think that the first Predator movie is better than the sequel...though not by much.
