Are we talking about objective quality in a thread about which sequels we think are good even though the world says they're shit?
Which sequels!?
...okay!
I'll throw in my vote for Rescuers Down Under, although aside from the new Winnie the Pooh it's the only non-Pixar sequel Disney has actually put in theatres. It also came out during the Disney Renaissance and is largely credited for saving the company from financial ruin. So I don't know if it's underrated...
Clerks 2 was almost universally hated and I'm glad I'm not the first to champion it on here. I thought it was an excellent follow up to Clerks, and it avoided the trap of trying to be too much like the first film. I might be one of the only people I know who has the Clerks Animated Series on DVD, and what I liked so much about that was how absolutely ridiculous it was, and was such a huge departure from the original (it was a kind of proto-Clone High). So I didn't go into Clerks 2 expecting anything but a movie about assholes talking shit, and what I got was a charming romcom that maintained all the thematic grace of its disillusioned predecessors. It was the absolute best that a sequel to a cult classic could be.
Ya, ya, cool story bro etc.
I liked The Lost World: Jurassic Park. So sue me, I haven't seen it in years and I'm sure it's awful, but I thought that movie was just fucking awesome when I saw it. It satisfied me on all counts: More smarmy Jeff Goldblum, more dinosaurs, less heart-warming kid scenes. It's not, you know, Jurassic Park, but DINOSAURS.
Another movie that I probably shouldn't mention is An American Tail: Feivel Goes West. Featuring James Stewart's final performance as a cartoon dog mayor. Wow, even typing out that sentence makes me doubt that I ever should have liked that movie. Let me watch it again and get back to you...
Tron Legacy was good. Other than that, I disagree with all of you. Also, I'm glad no one has said Advent Children yet, and I hope no one ever does.
edit: Oh yeah, and about Toy Story, just tonight my roommates and I rewatched TS3 (all in our mid-20s) and we bawled like little bitches. Again. Most people I know also cry during that Sarah Machlachlach(?) sequence in TS2, and I've never heard anyone with a soul utter a bad word against that trilogy. So you must be a bot.