Battlestar Galactica (the reboot) is up there for me. Gets written off because it was on Syfy which is pretty uniformly awful, but BSG was a top-notch show. It was like Lost + The West Wing + iRobot, in space. It was made at the perfect moment, encapsulating so many of the post-9/11 fears about security, politics, religion. They did a lot with a very low budget, and should be commended for it.
Lost, despite being a network show, was really engrossing and inventive and influenced a lot of cable shows that came after it, so I have to give it a nod even if it was ultimately a bit unsatisfying.
The first three seasons of Downton Abbey were quite good.
Mad Men, Breaking Bad, Orange is the New Black.