I'm in the middle of Preacher, jesus I need to finish it. Also Garth Ennis wrote the Max Punisher series which is awesome. My favourite of all time (so far) is Transmetropolitan, but I also love Y:the Last Man and Fables.
Also:
Ronin
Watchmen (although the movie dumbed it down)
Lucifer
Sandman
A History of Violence
V for Vendetta
Walking Dead (it gets tedious at times though)
There are so many more I want to read just don't have enough money to buy.