V for Vendetta, Maus (only comic book to win a Pulitzer Prize), I enjoyed Superman: Red Son and Wanted, but they're a bit more shallow than Moore or Speigelman. I also liked the Death of Captain America, but you may have to read some of the rest of Ed Brubaker's work as well as the Marvel Civil War to understand it. Sandman is brilliant. Anything by Neil Gaiman is brilliant. There's a comic adaptation of Neverwhere that's pretty good too.
And then, if you're one of those elitist snobs who only fancies "Graphic Novels" and ignores all the comic books, you can check out the really heavy depressing stuff like Black Hole by Charles Burns (which is FUCKING CREEPY), Fun Home by Alison Bechdel (it's a memoir) or the Adventures of Jimmy Corrigan by Chris Ware (who's just about the world's biggest tool). I don't necessarily endorse or like these books, I just have no idea what your taste is. You used the words "juvenile medium" in reference to comic books, so you could be an English professor.
There's a book called "Understanding Comics" by Scott McCloud which I recommend as well. It's kinda like a comic textbook, and it can help whet your appetite, as well as help you respect the medium.