The terrible movie from my childhood? The live action Super Mario Brothers movie. Only worth seeing in a group of people who care more about having fun watching the movie with each other than they care about watching the movie, really. It has its funny moments (most of them involving Dennis Hopper), but is otherwise exactly what I've come to expect from Hollywood adaptations of video game franchises.
To be fair, though, I -did- like the movie as a kid. I don't know why.
As for the good movie, it's Young Frankenstein. It's an affectionate parody of old horror movies, starring Gene Wilder as the grandson of Doctor Frankenstein. After a series of unlikely events, he ends up repeating his grandfather's famous experiment. I'd say hilarity ensues afterwords, but really, the movie is funny right from the start.