1. Mass Effect 3: 556 hours
I only played the single player once (incl all DLC), which took me around 30-40 hours. The rest is all multiplayer. Needless to say, I love this MP.
2. Skyrim: 421 hours
~99% of it just one playthrough. What kept me playing? First, the gameplay-borne immersion (not much can compare to that sense of wonder when playing Skyrim for the first time), and after that, mods. Mods everywhere.
1. KotOR 1 & 2
These games represented my entire being for a year or two when I was a kid (mostly K1). Ate, breathed, and slept it. Had to rip myself off - cold turkey - because I was so addicted. It felt like drug withdrawal (and most likely, was. [neurotransmitters/dopamine]).
I imagine I spent many hundreds of hours in these games.
2. Jedi Academy
This was the first, and for a long time, the only 'proper' game I owned. Played the singleplayer so many times that I lost count, and eventually grew to be sick of the it. After time has past, fond memories of it remain. I load it up every now and again to nostalgia; no matter how long I leave it, when I play it, all the map layouts and muscle memory comes back, as if I never left it.