My Personal Top10:
1. Six Feet Under - because it impacted me the most.
2. Breaking Bad - because it's brilliant, well executed, well thought out story from beginning to the end.
3. Banshee - this show has really consistent and super interesting aesthetic. Besides it's probably the best action-drama on TV right now if not the best ever. Don't expect realism though. It's more like a dark, gritty and slightly kitschy comic book with a lot of sex and violence on your TV screen.
4. Entourage - pure fun.
5. Fargo - this show is uber climatic. Creators did Coen brothers justice and then some. Oh and Billy Bob Thornton gives one of the best performances I've ever seen on screen.
6. True Detective - speaking of the best performances ever... Matthew McConaughey steals this show. Really tight story, solid character arc with a touch of a lovecraftian horror of the unknown.
7. House of Cards - another great performance by Kevin Spacey. Best political drama out there.
8. In Treatment (1st season) - this one certainly isn't for everyone. Each episode is a 30miniute conversation between 2 people. The viewer is basically a fly on the wall in a room where psychotherapy session takes place. What makes this show amazing are well written and acted characters.
9. The Sopranos - this show popularized the concept of anti-hero and for that reason alone should be on most TV enthusiasts top 10 lists.
10. Buffy: the Vampire Slayer - this might raise some eyebrows but this show was really ahead of its time. It's a really enjoyable story that's about progressively more and more important subjects as seasons go by. Also Joss Whedon's amazing dialogues.
Honorable mentions: Daredevil, Game of Thrones, Hannibal, The Newsroom, The Fall, Luther, Suits, Battlestar Galactica, Community, Mad Men.
I haven't watched The Wire and The Shield yet so my Top 10 might change a bit in the future.