Most Disappointing Sequel?

Most disappointing:

both Matrix sequels. Blech! Way to ruin a good mythos, Wachowskis
The Lost World: Jurassic Park II. The book was an obvious cash-in by Crighton, but it could have made a decent movie. The movie version just reeked.
Hannibal. Another case of an author cashing in. Once I finished the book, I knew the movie would suck. They changed the ending a little, but it was still crap.
Prequels, not sequels, but the less said about Star Wars: Episodes I-III, the better.
Star Trek: Nemesis. So was that crap at the end with Data's prototype supposed to be something like Spock's mind-meld with McCoy in ST II:TWoK? We'll never know, because Nemesis effed up so bad it killed the franchise and led to the JJ Abrams reboot of ToS.

Now for some sequels to good movies you knew were going to suck because these movies in no way demanded sequels:

Major League 2. Really?
Caddyshack 2. No, really?
Blues Brothers 2000. They saved the orphanage, went to jail for their vehicular carnage, and played some kickass shows in the prison dining hall. Did we really care what happened next?
Jewel of the Nile. Another movie (Romancing the Stone) where everything fit in perfectly, and it ends with the hero and the dame sailing (sort of) into the sunset. Why was this necessary?

And now, sequels to movies that were unexpectedly enjoyable.

Aliens. Instead of trying to recapture the feel of the original horror film, Jim Cameron created a slam-bang action flick. Too bad the franchise went off the rails in later installments.
Terminator 2. Another Cameron success. The less I think about the time travel implications, the better (hey, Skynet: send your upgraded terminator to the 1930s and kill Sara Connor's grandfather or something, it would be much easier than sending your cyborgs back to a time period where technology is catching up to them), but the action comes so fast you don't have time to worry about it.
Crocodile Dundee 2. No, seriously. The first one was a barely-interesting fish-out-of-water romantic comedy, the sequel was more of an action-adventure. Implausible as hell, sure, but Paul Hogan's personality helped carry it.
Austin Powers: The Spy Who Shagged Me. I really didn't care for the first one. The second one was better, in a "I'll stop channel surfing when I come across it" way.
Predator 2. A totally unecessary but enjoyable sequel, and it has Bill Paxton and Gary Busey. That alone can make a movie entertaining.