Planescape: Torment. I'm at about 8 playthroughs, IIRC.
A few good playthroughs of Arcanum, as well. And I still have quite a few character concepts to try out there.
Devil May Cry 3 3-4 times from new games to finishing Dante Must Die mode. And the bosses are still challenging.
Portal, though that's partially due to watching other people playing it through.
Thief 1 and 2 a few times. They've still got unique gameplay, and the setting is very much worth it.
Final Fantasy Tactics for... I dunno exactly how many playthroughs.
A few good playthroughs of Arcanum, as well. And I still have quite a few character concepts to try out there.
Devil May Cry 3 3-4 times from new games to finishing Dante Must Die mode. And the bosses are still challenging.
Portal, though that's partially due to watching other people playing it through.
Thief 1 and 2 a few times. They've still got unique gameplay, and the setting is very much worth it.
Final Fantasy Tactics for... I dunno exactly how many playthroughs.