Hmm. I usually tire of single player games after a while, regardless of how addictive they are.
I believe 110 hours with FFXII means it qualifies as "slightly obsessed on", but my characters are still only level 45, and I've got most of the optional end-game left to do. It will probably be tricky, too, considering I'm running out of stuff I can do at this level. Guess I need some more of those experience boosting accessories.
A more likely contender for me is Braid. By comparison I've only spent 30.6 hours with it, but remember that your first run usually only takes about three or four hours, tops. I've gotten every Steam Achievement (well, only one of them is prestigious) and the hidden ending, and run through the game... loads of times.
Cave Story doesn't really qualify - even though I've played through it countless times, it's often been to fill, say, bus rides, long breaks or boring classes. I regret nothing, however.
Now, Final Fantasy Tactics Advance: here's a real contender. I played through it once, and at around 80 hours /played (I think) I realised I shouldn't've chucked that Black Thread earlier. I restarted the game and spent another 70 or 80 hours doing it right this time, eventually getting 100% completed. I really should pick up #2 one of these days.
There's also been a bunch of games I've played for longer than I should have thanks to challenge modes, Achievements or both. Arkham Asylum comes to mind, with 30 hours of /played time and only two Achievements missing.
Mulling over the candidates, I think I'll declare it a 3-way tie between FFXII, FFTA and Braid. I'm done with Braid and FFTA, but I felt I got a lot more fun out of them than I should have, and FFXII, well, maybe I'll go for the 12-hour boss, maybe I won't. But it's just a brilliant game that's difficult to let go.