I'm gonna go with Buffy the Vampire SLayar. This show shouldn't have worked. It was based on Kristy Swanson movie, and it was a mid-season replacement- on the WB!! And yet, it lasted seven seasons, spawned a spin-off which lasted 5 seasons, and has had comics, novels, video games, RPGs, etc. They deftly managed to blend comedy, drama, & horror.
