Playing Freespace open source project- easily 4+thousands of hours over the last 20 years
next up would be World of Warcraft from 2004 to 2005, I was addicted easily playing 4-6 hours a day, that would put it just under 3000 hours during 1.5 years I was playing.
Then after that would be Guild Wars 2, I am 850 hours since release of that game which was August 2012