Ol' Bruce Wayne
I have to admit. This is the best comic book I have read. Ever. I didn't want to necessarily agree with Drei right away when he said this is the best ever, even before reading it.
Besides, I have come across some pretty good comic books during my time: Infinity Trilogy, Adam Warlock and the Infinity Watch, X-Men: Omega Red Series, Punisher WarZone, Legion Quest, Identity Crisis, Stormwatch Series, just to name a few.
But the Dark Knight Returns comic book has it all. Story, art, character development, scope. There are also a few things I have never seen before on a comic book or a story told that way (spoilers ahead) i.e. Superman's "Where?" message and the added focus on his "super-hearing" ability, the idea that heroes are what makes villains villains (no superheroes, no super-villains), Batman as ruthless f@#k, and children actually killed by the Joker and then his "twist" at the end.
Yep, this book actually takes the cake.
(image from Wikipedia.org)