There are some movies with such awful sequels that I had forgetten how good the originals were. I stumbled upon Revenge of the Nerds last night and couldn't make myself change the channel. It's one of my favorite stupidly funny movies, but the sequels were so awful and so numerous that I had even boycotted the original.
Other movies with awful sequels:
Saturday Night Fever
Planet of the Apes
Weekend at Bernie's
The Godfather (the third one, of course)
Indiana Jones (some were good, some not so good, none as good as the first)
Nightmare on Elm Street