Boardwalk Empire is another good show.
That's HBO though. As is Sopranos, The Wire, Deadwood, Rome, Generation Kill, Oz, Six Feet Under, Game of Thrones, Curb Your Enthusiasm and Band of Brothers. Almost every great show under the sun is HBO. Mostly other shows pale in comparison so I'm not sure if I have anything to really recommend that's not HBO. Breaking Bad is a piece of art but you've seen that already. The Walking Dead I thought was absolute tripe, I seem to be in the minority but that's what I think.
I see The Office UK is on Netflix. That's definitely worth checking out if you haven't seen it.