All-Star Superman is one of the greatest Superman stories. You may also want to check out Superman: Red Son, and the DC Stories of Alan Moore for some good Superman/DC stuff.
There's also Watchmen, Maus, Fables, and Hellboy. Those are great reads. They're all quite different, by the way. I'm sure you're familiar with Watchmen. Maus is a Holocaust memorial with different races and nationalities represented as animals. It's a very serious work.
Fables is an awesome urban fantasy where fairy tales come to our world. It's actually a rather adult series.
Hellboy is freaking Hellboy. It's great stuff rooted in mythology.