S1 & S2 of Parks & Rec, only because Paul Schneider was a regular. The show fires on all cylinders from the start, but any time he's in a scene it's a slog to get through. The "romance" storyline between Leslie and Mark is horrible and was upgraded 1000% when Adam Scott came on board and from then on, every episode and season was fantastic.
I was going to come in here and say the last two or three seasons of Dexter, but I didn't want to beat a dead horse. I've never went from liking a show as much as I did Dexter, to hating one so fast. After the Trinity Killer stuff, the show fell of a fucking cliff and was ate by sharks, then that shark was speared through the eye. Horrible, horrible, horrible.