Mine would shamefully have to be World of Warcraft... I can go into my three most played characters right now and tell you their combined play times.
My original main has a current total of 17 days, 17 hours, 22 minutes and 17 seconds.
My second main has a total of 35 days, 12 hours, 36 minutes and 44 seconds.
My current main has a total of 27 days, 18 hours. 59 minutes and 39 seconds.
So that's a total of just under 2000 hours.
And that's not including the time I've spent on my mountains of alt characters.
Oh dear.