Plenty of favourites already mentioned, so other Miller works then: Give Me Liberty, and its (first) sequel Martha Washinton Goes to War – both illustrated by Dave Gibbons who also drew Watchmen.
Graphic novels:
Sandman
Y: The Last Man
Fables
Manga:
Nausicaa
20th Century Boys
(both these have flaws, but overall are worth reading)
Batman: The Long Halloween is the only one I’d recommend that hasn’t been yet.
I basically learned Vietnamese by reading translated manga. The ones I read were:
Doraemon
Ranma 1/2
Inuyasha
Conan (maybe called “Case Closed” in English)
I like Doraemon the best, especially the first few volumes. To me, they’re still comics, and I want them to be funny and generally light-hearted.
Several good recommendations so far and I’ll add two of my favorites:
Astro City by Kurt Busiek and Brent Anderson. There are a few volumes, all excellent, but start at the beginning with Life In The Big City. It’s a fresh take on some traditional superhero archetypes, breathing a new sense of wonder after the “grim & gritty” era.
The Chuckling Whatsit by Richard Sala. A black-and-white mystery that captures the look and feel of a slightly twisted noir film from the '30s, with some mildly bizarre elements. Just brilliant.
Moonshadow, written by J.M. DeMatteis, is a whimsical fairy-tale adventure story, beautifully illustrated.
Frank Miller’s Sin City books. The first collection is now being sold with the title “The Hard Goodbye.”
V For Vendetta by Alan Moore
One more manga to recommend:
Cross Game
Probably better than the previous 2 I mentioned.
The Maxx by Sam Kieth, a surreal, trippy send up of superhero tropes that evolves into a deep and personal psychological drama. My favorite comic of all time. I don’t think all of it ever made it into Trade Paperback, but most of the run did.
Scud: the Disposable Assassin Their tagline says it all: “Surreality just got funky!”
Perhaps you might want to read some predecessors to graphic novels:
Thank you people. A lot to choose from.