Glad to see you've discovered Kirkman. If you like his stuff and the MU, definitely check out his Marvel Team-Up, and Marvel Zombies. His old series Brit and Battle Pope are also lots of fun.
Also from Image, check out Girls and Fell (INCREDIBLE book)
To be technical, Preacher and Y-The Last Man are DC. They're published through Vertigo, which is DC's Mature readers lines. It's hard to go wrong with any Vertigo title. Fables, Transmetropolitan, 100 Bullets, Sandman are all great series, and they have beautiful minis and graphic novels like Pride of Bahgdad, Orbiter, War Story, and others.
I would easily say Fables is one of, if not the, best titles on the stands today.
Wildstorm has some cool stuff, too, like The Authority, Planetary, and Ex Machina