Gun to my head, probably F•R•I•E•N•D•S. No gun to my head, smurf you there are too many great shows to choose from!
Shows that might've made it to be the best:
Life on Mars is great but its too short to watch over and over (don't get me wrong it's brevity is it's charm), and I try not to think about ashes to ashes
Community falls off a bit after season 3.
Game of Thrones has yet to finish but it could be up there.
Breaking Bad didn't grip me in the first season or so (though to be fair neither did Friends)
Futurama is great but the latest season has a few flaws which annoys me as does one of the movies
30 rock starts and ends with mediocre seasons
Stargate lost Jack O'Neill at the end. (I even liked Vala)