Okay, forget reading reviews and shizz, I'm gonna tell you what you will enjoy right here in this post.
You are exactly where I was a number of years ago. The Sandman was the first true graphic novel I read and fell in love with. It is probably my favorite of all time. I will repeat some suggestions made in this thread as they are pretty good. Although remaining in the Sandman universe, there are a bunch of comics about Death, but they're not that good. But definitely do pick up Sandman: Dream Hunters. It's a one-shot collaboration between Neil Gaiman and Yoshitaka Amano (he created many of the Final Fantasy characters and does wonderful art).
My next favorite would probably be Preacher. This thing is awesome, and is a great graphic novel to pick up after Sandman. I think it runs about 8-9 collected paperbacks (Sandman is 11). It's quite blasphemous but the premise is awesome. The story can be epic, hilarious, gory and sad all at the same time.
The Walking Dead is definitely worth a read. It's about group of people living in something of a zombie apocalypse. The story can be quite messed up. You should buy the bigger books for this one, they collect more issues and are kinda huge (I don't mean just page-wise, the design is pretty big - and the covers are hard not paper). The fourth one's ending really caught me by surprise.
Fables is pretty cool. The premise is basically that the worlds which fables inhabit are attacked by an enemy that drives them out of their homelands. They escape to our planet and build themselves a community in New York. It has really interesting takes on characters like Snow White, the big bad wolf, and others. It's a very fun read and I really enjoyed it (it did take me maybe 2-3 collected volumes to really get into it, though).
100 Bullets is damn awesome. The basics: A man, Agent Graves, comes to someone that has been wronged, and hands them a briefcase. The briefcase has evidence of why they are where they are, or why what happened to them happened, and who is responsible. It also has 100 untraceable bullets and a gun. It sounds simple, but the story goes much deeper than that and it is quite interesting. Brian Azzarello's writing is awesome, especially his dialogue. The whole thing has an awesome neo-noir feel and Eduard Risso's art just gets better with every collected paperback.
Ex Machina is also a lot of fun, and Nurgey described it pretty well.
Y: The Last Man is also pretty good. The last issue didn't leave a lasting impression, but overall I really enjoyed the series. It's the story of what would happen if every male on earth had suddenly died (except one man and his pet monkey), and their quest to save humanity.
The next thing I personally want to pick up is the Hellblazer series (about John Constantine). It seems to have had a lot of great people work on it, such as Brian Azzarello (100 Bullets), Warren Ellis (Metropolitan), and especially Garth Ennis (Preacher). Like all the suggestions I just mentioned (except for maybe Ex-Machina), it is published by Vertigo. The stuff they publish is usually more mature, and pretty damn good (Sandman included).
Now as for Sin City, I read it a while ago. I remember some issues being pretty good, and some being okay. You could probably finish the whole thing in a few short trips to the library hehe. Akira is manga. If you liked the movie, it's worth a look. It's not hard to find like someone mentioned. There are six huge, and fat collected trade paperbacks for it. I liked it overall, and it really expands upon the movie so much. However the one thing that bugged me is how Americanized the dialogue was (why does chrome insist I spelled dialogue wrong?). It's probably an old translation and they didn't try to keep true to the original source material as best as they could. But worth picking up as you're not likely to get another translation.
As for manga in general. I really haven't found anything I really liked. Overall I feel that American and British graphic novel writers do a better job as you don't really get as much filler material (if any), and since the story isn't so spread out, it's easier to find good quality stuff. The only Japanese manga I read at the moment is Berserk. It's my favorite anime and I want to know where the story ends. Worth watching if you haven't yet.
I promise you, you will not be disappointed by this list.
