Almost all of the horror ones qualify:
Halloween: the original is one of my all-time favorite movies...wow should that have stopped there, and for God's sake it should have stopped after he blew up at the end of the 2nd one. Halloween 3 I think gets the distinction as the single weirdest fucking "sequal" ever as it had absolutely nothing to do with the first 2. I won't even bother to comment on 4, 5, H20...or the Rob Zombie goat placenta of a remake.
Nightmare on Elm Street: Again, the first one was actually prety decent, relatively original story, creepy villain, a young Johnny Depp...the mother was a terrible actress, but it was a good movie nonetheless. It went seething downhill from there, and the craptacular volume of sequals has ruined the appeal of the original.
Friday the 13th: First one wasn't good per se, but was at least decent...not many remember it was actually Jason's mother that was the killer in #1. The 2nd was awful, #3 was kinda campy, and the rest were pitiful. Jason Takes Manhattan should get an offical Congressional Reprimand.
Hostel: Not a big fan of either movie, the ending of the 2nd one can be considered shocking only if you consider how much of your life and your money you won't get back watching a dog stare at a rubber dick.
Jurassic Park: Original was good, sequals were awful.