Deadwood was probably the most vulgar show I have ever watched. And yet the dialogs were perhaps the most poetic and intense that I have ever enjoyed. There was a Shakespearean quality to the several of the characters – they were carved in stone and yet so incredibly human. The sets were also incredibly realistic. The whole premise of a show based on a town that only existed for 4 years on the frontier of the American West was fascinating. Seth Bullock was memorable and reincarnated recently in Justified (at least according to my related post), but the character that steals the stage and captivates the audience is, of course, Al Swearengen. He is, perhaps, the most evil, vile, and violent character I have ever seen on television, and yet this exterior hides a more sensitive, human side which we rarely see but which endears the character to use even more.
I think Deadwood for me featured the best screenwriting for dialogs and some of the best sets of any of the shows that I have followed.