Batman: Year One, The Long Halloween, Dark Victory, Arkham Asylum, The Dark Knight Returns and The Killing Joke are all excellent. Hush is a bit predictable but very good. Knightfall is very good but frustrating. The entire story covers three graphic novels and once you hit the second book multiple issues are missing, making the story incomplete. I never finished the entire story arc. A Death in the Family is complete crap and I can't believe it gets good reviews.
As for the non-Batman comics that I've read: Watchmen is amazing but is a laborous read, Frank Miller's Sin City is very good, Garth Ennis' run on Punisher MAX is amazing, DMZ is good, 100 Bullets is good but largely forgettable. Last but not least: Preacher is the greatest thing ever written. Okay, maybe not the greatest thing ever written, but it is easily my most favorite comic series. I've ready it three times and I'm looking forward to the fourth.
I would love to see Preacher adapted for the screen if done properly. A few years ago Mark Steven Johnson, Garth Ennis, and Steve Dillon went to HBO with a direct adapation, using each issue of the comic as an episode in the show. HBO toyed with the idea until ultimately deciding the public backlash from the show would be too much. When HBO is worried about the content of the material, you know you're dealing with some crazy/amazing stuff.