Anything with Alan Moore's name on it is likely to be good. He's the genius behind Watchmen, the League of Extraordinary Gentlemen, and V for Vendetta. He also worked on the Killing Joke, one of the Batman's that allegedly lead to Heath Ledger's madness. He is currently working on the Neonomicon, a graphic novel based on H.P. Lovecraft's Necronomicon. I've heard these are pretty good but I haven't had a chance to pick any up. Alan Moore's work is generally a pretty high standard in the world of graphic novels.
Another great Batman graphic novel is The Long Halloween. Amazing artwork and story, features quite a few Batman villains, and makes Batman look like a total badass.
DC recently held a major DC Universe event called the Blackest Night. This series featured dead superheroes coming back to life to trouble the living. This is mostly a Green Lantern event, but almost every DC Universe character is affected by the Blackest Night, so if you have some favorite DC characters try to find their BN issues. They followed this up with Brightest Day which again, I haven't yet looked into.
On a completely different note, the Scott Pilgrim graphic novels are good in their own way. The illustration is low quality, and the story-line is often very jumpy, but they are always funny.
