I unfortunately never finished Y The Last Man, but what I did get through was great. That's one series I definitely want to go back and finish up. Hoping they make a fantastic tv series or set of movies out of it.
I just borrowed Preacher and The Walking Dead from some friends at work and they quickly rose to the top of my all-time list. Almost been like mental flagellation, each in their own ways: so much pain, but such great reads.
As for Sin City, I'm glad to find someone else who enjoys those books. It's hands-down my favorite comic-created world. I know the dialogue can be cheesy as hell, but coupled with stark black-and-white and the craggy lines of Miller's art, it just works. Coming out of Michael Madsen's maw or being butchered by Brittany Murphy, not so much. A lot of my comic-reading friends find it empty and Miller's writing horrible.
If you like the hard-boiled detective stuff, you should check out the Blacksad books. The art is fantastic and the author deals with social issues in each volume. The titular character is a black cat in a trenchcoat. I think there are currently three books.
I actually just picked up a copy of SiP because it was really cheap, but it's vol 6...i need to go to the bookstore and find the first few volumes. I've been told I'd like it.