It seems like comic book movies are one of the places where it's possible to have a sequel at least as good as the first one was.
Yeah, that's true. Probably because, for me at least, the "how the hero gets his powers" part of superhero movies is usually kind of boring, especially when it's a story I know well: Spiderman, I'm looking at you.
Anyway, two more, non-superhero variety:
Dawn of the Dead is a sequel that improves on an already great movie. I love the feeling of claustrophobia in Night of the Living Dead, but I love the more expansive scope of DotD.
Not technically sequels, but The Good, the Bad and the Ugly is better than For a Few Dollars More, which in turn is better than A Fistful of Dollars. But they're all good movies.