I'll say Under the Dome. It started out with an interesting concept (and the book was pretty good), and recently, it's just been more and more ridiculous. You know the writers are making it up as they go along.
Supposedly Under the Dome was only supposed to be a one-season deal, and the production team was completely blindsided when the network asked for a second (not so blindsided that they considered turning down the money, of course). But that's why the plot has gotten so random and disjointed - they're stretching things out on the fly.
I don't think anyone's mentioned the League yet, which I really liked for about 3 seasons but has since turned into a bloated caricature of itself. Parks and Rec didn't start off well but was great for a few seasons and is just circling the drain now. Oh and the Walking Dead - brilliant pilot, strong first season with some flaws, since then it's been pretty hit or miss with more misses than hits as it's gone along.