In terms of what game I've enjoyed most I would have to say the original Kingdom Hearts. Granted that came out back in my gaming hayday, so that might have had a huge part in it.
In terms of what game I've clocked the most hours on...Final Fantasy IX probably. I easily played that game 100+ hours. I want to re-play it soon, this time on Steam, but I'm not sure I have the time to sink into my obsessiveness.