It's actually one of the few sequels I can say that, hands down. No thought necessary.
I haven't seen Alien since I was probably 7? I only remember the basic concept of what happened. Aliens, however, I have seen a gazillion and one times.
I had a friend when I was twelve who hadn't seen Aliens and I was all *epic gasp* WE MUST WATCH IT THIS INSTANT!
So we did.
Or at least we tried to.
I put in Alien thinking it was Aliens.
I stared in horror at the screen, thinking, "This isn't right! This is horrendously boring!"
Which is always the worst thing to be thinking when you've praised a movie and then forced someone to watch it with you. I don't think we got past five minutes before I shut it off in a mix of outrage and embarrassment. And boredom.
So, yeah. Teen angst aside, I like Aliens WAY more.