Some that I either didn’t see mentioned in the earlier thread, or I’m seconding the recommendation:
Batman: Year One
My favorite “graphic novel,” because it’s so straightforward and solid. Tells the story of the origin of the Batman as a real story with real people involved. Not as some self-consciously gritty crime noir, or a fantastic super-hero story, or a heavy-handed psychological metaphor, or a hyper-violent action story. And I’m not even a fan of Frank Miller.
Any of them. Seed of Destruction and Wake the Devil are the “big” story, and The Chained Coffin and Others is my favorite of the trade paperbacks. Hellboy is my favorite series because the author just “gets” it; it’s just the right combination of horror, action, comedy, mystery, and mythology, all told with beautiful art and minimal dialogue.
Mage: The Hero Discovered
I loved this one in college, but re-read it recently and didn’t like it nearly as much. Still, it’s got a big fan following. It re-tells the story of King Arthur in a contemporary (at the time – late 80’s to early 90’s) setting. It’s clever, imaginative, and suitably epic, but now it can feel kind of talky, obscure, and self-important. Your mileage may vary.