Pretty hard to recommend without knowing your taste/what you're looking for, but most of the good ones have already been said. Probably my favorite series are (in no particular order):
Sci-Fi/Fantasy:
Babylon 5 (really cheesy to start but just keeps getting better and better)
Firefly (Its cancellation is still one of the great crimes of the new millennium)
Game of Thrones (Just don't watch it with the kids)
Drama:
House (The 2nd and 3rd seasons are just pure gold, great mix of cleverness, humor, drama and medical mystery... things start to go downhill around season 5 but it's still worth watching just for Hugh Laurie)
Boston Legal (Great mix of legal drama & sillyness, James Spader and William Shatner are great together)
The Wire (Pure Excellence)
Sitcom:
Spin City (Great series until Michael J. Fox had to leave because of his Parkinson's. Didn't work too well after they replaced him with Charlie Sheen)
Big Bang Theory (Because geeks are funny)
Frasier (a classic, consistently funny throughout its 11 seasons)
Arrested Development (slow moving, but the quirky humor snowballs every episode, only 2 1/2 seasons)