Oh, golly gee, it's a Versus Thread. So much fun...
Anyway, sarcasm aside, it's Aliens for me. While I admire Alien for being an excellent film, I just wasn't feeling it with the whole horror thing. It may be that I didn't watch it in a very scary environment, but I just didn't find the film scary. Tense? Yes. Scary? No. Also, my dad decided to summarize the entire plot for me as a kid (which he also did with other horror films), so I knew what to expect. He even ruined the chestburster scene.
Aliens, however, he didn't really spoil, so when we watched it, all the twists and turns of the plot were new. It also does the whole action/horror thing very well, making for a tense, gripping experience.
Alien 3 I hated. It wasn't a bad movie, but it just doesn't fit the Alien series very well. Alien 3 was originally going to be about a war between the Xenomorphs and humans on Earth, which would have been awesome, but never ended up happening.
Alien: Resurrection was stylistically really cool, and was more of a return to form, but Alien 3 had already killed the franchise, and Resurrection had enough issues that it couldn't revive the series on its own.