Gaiman's Sandman
Carey's Lucifer
Kingdom Come
Moore's Watchmen
Spiderman: Kraven's Last Hunt
Carey's run on Hellblazer
Ultimates
Wolverine: Origin
PREACHER, bitch.
Marvel Zombies

I was tempted to put a lot of Elseworlds titles in my list since I found a lot of them fun to read but I realized that they have more impact if the reader is familiar with the character's continuity. I'm gonna wait until my friend is more knowledgeable about character histories before I get him hooked on EW stuff(technically, M. Zombies and Kingdom Come are of the same type, but they seem to stand as good reads by themselves)