Off the top of my head...
"BloodRayne 2: Deliverance." Not that it was much of a drop in quality, but I actually enjoyed the first one. The sequel bored me.
"Hollow Man 2" I don't even remember much of it. But once again, not a big drop, but I liked the first one and hated this one.
"Lawnmower Man 2" Yeah, probably not much of a drop either, but to me it is. I liked the first one for what it is. The second one doesn't even know what it is: it's cheesy and kid-friendly one minute and grim the next. Terrible (and unintentionally funny) dialogue and acting. It's lower than a B movie.
"Superman III" The first two were fun! This one (and #4) is just way too campy, goofy, and dumb.
"S Darko: A Donie Darko Tale." Probably the biggest drop in quality IMO. The first film is brilliant! This DTV sequel seems to rehash the same stuff to tell an uninteresting, confusing, irrelevant tale.
And ala my avatar, "Heavy Metal 2000." I think it's a pretty fun sequel, but I gotta admit that the story's not that great. The first film is better.
