Seconded for Baccano!, Durarara!! is pretty good too.
Cowboy Bebop. I'm not going to stop saying this until someone who hasn't watched it watches it. If anyone uses the "I only like recent anime" excuse, I will drop my pants and stuff on them.
You can try watching Gundam Unicorn. It doesn't absolutely require knowledge of prior Gundam series since they do explain most of it. If you want to understand the entire thing from beginning to end, it needs that knowledge. It's still airing, the last episode comes out next year.