"Luthor" by Brian Azzarello and Lee Bermejo
Brian Azzarello is my second-favorite of the contemporary comic book writers, standing - in my estimation - just behind Mark Millar. I think that one day he will write an even better comic book, something that people will be unable to ignore, and I might just be calling him my favorite.
He is great at telling the villain's side of the story, and in terms of grittiness, realism, and moral ambiguity he is beyond reproach. He tells stories that are just a half-step removed from our own, and presents characters that exist on the borderline between heroism and villainy.
I read his and Lee Bermejo's "Joker" a while back, so I was already familiar with both of these talents before I started "Luthor." "Joker," I think, is slightly better than "Luthor," but both books are excellent. "Joker" stood out for its shockingly original violence, while "Luthor" stands out for its aphoristic majesty, and its portrayal of the arch-villain as a misunderstood humanist. "Joker" was something that sat with me for a long time, while I feel "Luthor" is a bit less memorable.
Anyone unfamiliar with Brian Azzarello and Lee Bermejo's work should certainly seek it out. You won't be sorry that you did.