I pick Superman first, then Superman II. The others aren't even in the same league.
I've watched the remastered Superman DVD quite a few times in the past year. Well, the good scenes from it. It is full of great parts, and Williams' score is his best IMHO. I like just watching Superman I's opening space scene credits because of the music. Good Lord, I can still remember seeing it at the theater as a kid about 10 years old.... that music stirred me. It still does. The thing I like most about S1 is the awe that is reflected in people, as Superman is an unknown entity and he defeats everything.
Superman II is really close though, it really is good. I loved the revenge-on-the-bully scene and the Kryptonian fights. The movie is more rewatchable than the first, I think, because its faster paced & has more action.
Superman III and IV both are crap, but Superman III does have some really great scenes, like the Evil Superman stuff, and I like seeing Lana Lang too.
I only own Superman I, gotta find Superman II on DVD somewhere. I haven't seen it for a few years.