The West Wing and The Wire both immediately come to mind as excellent. I'm a huge fan of Sorkin's writing (c.f. The Social Network, albeit with a most unappealing subject!).
TWW later series (after he stopped writing for it) are nothing like as good. We're just into series 2 on DVD, as youngest daughter wants to be a lawyer in the USA! She's finally realised The West Wing is something you have to concentrate on (not the vacuous pap she usually watches!), and she's really getting into it. I'm delighted for her - topics to argue about, great dialogue, altogether in a different class from, say the "three musketeers" (I was in the room last night, not pleasant).
Am I a grumpy(-enough) old git?
E.