On the drama side, I have to put Justified at the top of the list. The first season established an excellent tone and the writers' knack for writing sharp dialogue, but it lacked a bit in terms of serialized week-to-week drama. However, after that first season the show has really taken off and not looked back. The Shield is a close second for me, and stands out as having possibly the best series finale ever. Damages, Rescue Me, and Sons are also highly watchable shows, but they just have never really clicked for me. In the case of Rescue Me and Sons, I think those shows to a large degree just have/had outstayed their welcome and at times the plot lacked any real direction.
Also, I'd be remissed not to take this opportunity to mention Terriers. Great show with awful marketing that deserved to have more than one season.
As far as comedies go, Archer and It's Always Sunny are two of the best on TV. The League is also pretty good but some of the characters can at times be a bit grating.
Louie is bottom line one of the most fantastic things in all of television, and I'm constantly amazed at the fact that FX continues to give Louis CK the money and freedom to do it. This seems like exactly the kind of show that would typically get focus grouped straight into cancellation, and yet somehow the suits just continue to let Louis CK do his thing on a shoestring budget. It defies conventional labels of drama or comedy, and instead delivers something entirely its own, often resembling an avant-garde short film more than an episode of a sitcom. Reality shows claim to show real people in real life scenarios, and yet none of them that I've seen captures real life the way Louie does.
That show is so real and funny. Although it's not a 1:1 account of Louis CK's actual life, it feels that way while being hilarious simultaneously.
Close second is Always Sunny which is the exact opposite. It's like they finally figured out a way to turn an animated series into reality.
Lotta good shows on that list but Rescue Me deserves some love. I don't know if I'd say watch it instead of some of that other stuff but if you're looking for a new show to watch, it's real damn good. Dennis Leary is awesome and it's a great show about NY firefighters that deals with some post 9/11 stuff without coming across as shmaltzy. Great cast.