Note: I haven't seen The Wire, which I'm told I'd love.
Law & Order (1990-present). Though sometimes heavy handed and formulaic, this show has provided countless hours of good drama. Crime/Mystery/Drama.
Will & Grace (1998-2006). In spite of its self-consciousness, this sitcom's humorously self-centered characters make the show amusing and lovable. Comedy.
Angel (1999-2004). A supernatural soap opera that is angsty and epic. Action/Drama/Fantasy/Thriller. (Top 6.)
The Office (UK) (2001-2003). A biting, brilliant satire set in the working world which is dark, realistic, and absolutely hysterical. Comedy/Drama. (Top 6.)
Six Feet Under (2001-2005). A beautiful and therapeutic take on life and death. Revolves around a family-run funeral home. Drama. (Top 6.)
Firefly (2002-2003). This funny, exciting, and original space Western features a fantastic crew of characters. A pure delight. Drama/Sci-Fi. (Top 6.)
It's Always Sunny in Philadelphia (2005-present). While the recent episodes have been more strained, this series about stupid, terrible people includes some of the most hilarious stuff I have seen. Comedy. (Top 6.)
Mad Men (2007-present). Occasionally too composed, self-satisfied, and unfulfilling, this show about 1960s "ad men" is still very intelligent, at times scary, moving, and comical. Drama.
Dollhouse (2009-present). As far as I can tell, unlike any show ever made. That's a compliment. Extremely thought-provoking, multi-layered, and intense. Drama/Sci-Fi/Thriller. (Top 6.)
The Vampire Diaries (2009-present). This entertaining teen drama (with ghostly and vampiric danger thrown in) is like tasty junk food. Good for what it is. Drama/Horror/Romance.