I can understand why people think The Godfather Part 2 is better (even though I stongly disagree), but I have never understood how anyone can prefer Aliens to Alien. The original is one of the most tense, claustrophobic and frightening pieces of filmmaking I've ever seen, the second one just seems like a slightly dumbed down action/sci-fi flick.
Anyway, I can't really think of many superior sequels, off the top of my head, only Blade 2, The Road Warrior and X-2. I would say Kill Bill Vol.2, but that doesn't really count seeing as it's just the second half of one movie.