At the moment, it's Final Fantasy XIII.
I try and stop myself by thinking about the bad points of it, like how all the things left to fight are too hard and that all I'm really doing is grinding, but by the time I've done that I'm already playing.
And once I start, that's another three hours of lifespan down the toilet.
TF2 as well, but only when I'm not doing badly. I've had a good run with the Demoman lately, so it's been drawing me back pretty frequently.