Some really good ones mentioned (Highlander II, Nightmare on Elm Street II - the scene where the bird explodes is awesome), but I'd have to say Troll II is the worst sequel ever made:
Notable in part for not featuring any trolls (the antagonists are goblins from the town of Nilbog — which is goblin spelled backwards), the film also has no relation to the original Troll, which was also critically panned. Not only one of the "least scary horror movies ever", according to Yahoo! Movies, but "by pretty much any measure... one of the worst films ever made". Director Claudio Fragasso (who used the pseudonym Drake Floyd for his work on the film) has maintained for twenty years that the film is a "masterpiece". Despite the script being written in awkward language (Fragasso and his wife Rosella Drudi, native Italians, spoke virtually no English when they wrote the script), Fragasso insisted the American actors deliver the lines as written. The goblins in the movie are dwarfs wearing burlap sacks and latex masks. Campy acting, confusing plot twists and unintentional homosexual innuendos have contributed to give the movie a cult status comparable to The Rocky Horror Picture Show. Nearly twenty years after its release, the movie's child star, Michael Stephenson, made a documentary about the film titled Best Worst Movie, released to critical success in 2009.
Any of these would make perfect MST3K fodder.