For me it would be The Following. I thought season 2 was abysmal, but I'll still be there on March 2nd for the season 3 premiere. Not sure why lol, but I continue to watch it.

Honorable mention goes to American Horror Story.

When LOST was on the air I watched it for far longer than I should have...

True Blood. I used to watch it with my ex. I tried to watch all of it, since I don't like to leave things unfinished, but I only managed to get through the first four seasons before I quit.