I'm seeing some interesting recommendations...
The comics that come to mind for me...
Watchmen
Maus
Daredevil: Born Again
Batman Year One
The Dark Knight Returns
X-Men: Days of Future Past
Amazing Spider-Man: The Night Gwen Stacy Died
Superman: Whatever Happened to the Man of Tomorrow?
All-Star Superman
Fantastic Four: The Galactus Saga, This Man, This Monster (I consider the epilogue part of the larger story.)
Sandman: Season of Mists
Some relatively obscure favorites...
The Adventures of Barry Ween, Boy Genius: Monkey Tales #4-6 (The sense of humor and character really appeals to me; This was the highlight of the series.)
The Best of Will Eisner's The Spirit (It's not that Eisner's Spirit is badly regarded; This TPB is just such a good intro to it, although there have been some comments that the volume was selected more to avoid horribly dated racial caricatures than by any other criteria.)