The entire run of Preacher. I only have the first three trades and haven't read the rest yet b/c I'm poor, but what I've read so far makes me feel safe in recommending the entire thing. As long as you're not easily offended, that is. Graphic sex, violence, deconstruction of religion, foul language and a character named Arseface... something for everyone!
The Boys is also great for that, and again, reading the entire thing from the beginning is the only way to go.
All the Superman and Batman stuff people recommended is solid, but I disagree about Grant Morrison and Jeph Loeb, I think they're awesome MOST of the time but more in a fanboy-pleasing, widescreen action movie kind of way, not so much as far as in-depth character stuff. Although Loeb's "Superman For All Seasons" and Morrison's entire Batman run have plenty of character stuff.
Morrison's "Batman RIP" is great if you read it and pay attention. "Final Crisis" wasn't everyone's cup of tea, and you kinda need a PhD in DC to fully understand and enjoy it, but I thought it was brilliant once I read it all at once (after hating it when I first read it, go figure).
Booster Gold is the best comic book dealing with time travel that anyone has ever done.
"The Man of Steel" and "Superman: Secret Origin" are both great. Two different versions of the Superman origin, both supposed to be "definitive" with plenty of conflicting points, but honestly, they're both so well-written that it doesn't even matter. "Superman: Birthright", however, was ass and it's not surprising they don't mention it anymore.
"Zero Hour" and "Infinite Crisis" are awesome stories about alternate Earths and parallel timelines. They're both sequels to the original "Crisis on Infinite Earths", but I honestly find them more streamlined and enjoyable than the original, and you don't need to have read it to follow either of these sequels. "IC" in particular sums up the entire plot of "Crisis" in like three pages.
"Green Lantern: Emerald Twilight", "The Final Night", and "Green Lantern: Rebirth" chronicle the fall and redemption of Hal Jordan, the preeminent GL of Earth. Awesome stuff.
"Guy Gardner: Reborn" and issues 1 through 16 of his own series are highly underrated, especially the "Yesterday's Sins" arc around issue 10 or so, the "secret origin" of Guy Gardner and the trauma of his childhood that added tons of depth to a character mostly used as a one-note joke.
That'll do for now, I don't wanna be here all day.
