Buffy the Vampire Slayer. No question. Period. End of story. ALL-TIME FAVORITE. And always will be.
I do have close runners up though, including Gilmore Girls, Supernatural, Veronica Mars, and Buffy's awesome-beyond-words spin-off, Angel.
Dark Angel
Teen Titans
Bleach
Scooby Doo
The X-files
Sanctuary
A Haunting
Ghost Hunters
Dog the Bounty Hunter
The Troop
Walker, Texas Ranger
Courage the Cowardly Dog
The Golden Girls
The Steve Wilkos Show
Angel
Supernatural
Gossip Girl (me & My sister are so addicted, been that way ever since the very 1st ep)
Gilmore Girls
One Tree Hill (Seasons 1-6)
Teen Titans
Buffy the vampire slayer
Angel
George Lopez
& Anime plus Asian dramas
Other than that the tv will be abandoned unless when im playing video games
My favorite is definately Law & Order: Criminal Intent but I hate the new episodes because I miss Goren and Eames. However Fringe is a close runner up and I sometimes tell people it's my favorite because I watch the new episodes. Also I really really love NCIS:LA because they have great team dynamics and a hot male star.