Game I played most is Knights of the Old Republic II.
Not surprising seeing how I work on the Restoration Mod for it, testing takes quite some time and replays.
Game that keeps me getting back all the time is Majesty, a 2000 game I occassionally do again from time to time.
DKS probably has no chance to beat either, but it's a very good game in it's own right. Not many games I wish to replay instantly after finishing... Although many games prevent me from finishing it now...