Bone, by Jeff Smith, is way up there. Anyone who hasn't read it, go pick up the one-volume edition. It will make your life better.
Understanding Comics by Scott McCloud will too.
Berserk by Kentarou Miura is the best, most epic, most soul-crushingly dark comic there's ever been.
20th Century Boys by Naoki Urasawa is the best, most epic, smartest, most best, supergreat thing, I'm running out of superlatives. But everyone should read this too, its the best mystery anything I've ever read. It will turn your mind inside out.