I've spent the most time on CounterStrike: Source, although I don't play it much anymore. Most recently it'd be PES2009 (Become a Legend mode). However! I've just got Virtua Tennis 09 and Fuel, so I shall be spending a number of hours on them (mainly VT09 I imagine).
Edit: I used to be quite addicted to MineSweeper aswell, but since my eye sight has gotten worse and my reaction time with it, I've been clicking on the wrong squares accidently more often so I only occasionally play it now.
2nd Edit: I forgot about FIFA Manager 08, I remember spending loads of time on that. According to my Xfire profile I've spent over 900 hours on that. When 2010 comes out I think I'll end up clocking around 1000 hours on that :/