Jon,

Depends partly on what you’re looking for, of course.

On superhero comics, I don’t have much to contribute other than what’s already been said.

In the broader field of comics, I really, really strongly recommend Marjane Satrapi’s Persepolis books, Alison Bechdel’s Fun Home, and Dykes to Watch Out For (you might want to wait for a while on the last before buying; there’s supposed to be a big collection coming out this fall, with most of the run collected in one volume rather than spread out over 11 different (varyingly difficult-to-find) books.