Only ones I've seen:

22: Teen Wolf Too- really, really bad
20: Godfather 3- not that bad at all, just too long and not as good as the first two. Beautiful looking movie, IMO.
14: Conan the Destroyer- more silly and fun than the first one, but not completely terrible..
12: Star Wars: The Phantom Menace- awesome production design and the pod scene is damned cool. Definitely had some good parts...
5: Batman and Robin- truly awful
2: Caddyshack 2- really, really bad. Deserves to be here.
1: Staying Alive- one of the worst movies I've ever seen in many ways. It's almost hard to believe this isn't a satire.

Others to consider:

Texasville (sequel to The Last Picture Show). has some redeeming qualities, but up against the masterpiece of its predecessor, this is pure shit.

Superman 4: The Quest for Peace. The first two were great, the third one not that bad. The fourth one? Ugg.

LOL, we got that for free in the double feature disc and I have absolutely no intention of ever playing that side.

Technically, Star Wars: The Phantom Menace is a prequel, not a sequel. EW, don't you have fact checkers in your offices?

I enjoyed Ocean's Twelve. I did not like Jaws: The Revenge, Scream 3, and The Klumps.

Well, I guess they should've made a top 100 worst sequels list.