Probably either Pokemon, Animal Crossing, or Left 4 Dead.

Team Fortress 2, 1825 hours. Pretty much what I did when I was in high school.

I've said this already, but, I'll say it again just because the reactions are hilarious. Longest I've played a game is around 24 years. My character's played age is 65,982 hours, or 2749 Days, 6 hours, 41 minutes.

RuneScape - 11200+ Hours. Still adding some time to that via Old School version of the game.

TF2 with 424 hours in total, I've got too many games on the Steam list to check the rest but TF2 seems to be the highest in memory.

Though now I think of it I believe the longest ever would have to be WoW since I started that back in 2004 and though I've been on and off it I'd think the total time would be well in the 2000 range if not more.

Pokemon Emerald. 300+ hours. I don't have the exact number as I deleted that save file to move on.

osu! was at 2000-3000 hours when I stopped using Xfire to track my times, I think. Should be 4000-6000 hours now, I think.

Super Mario 3D World... Like 90 hours.

Longest time in one session was about 15 hours on Paper Mario on the GameCube.

Star Ocean 3... about 600 hours. Obviously I am probably a bit more excited about Star Ocean 5 than most, and much more than I am comfortable admitting. 

About 400 hours in TF2.