Steam didn't track hours when I used to play CS1.6 but I put hours and hours and hours into that game.

Other than that: Zelda Link to the Past (it was my Elder Scrolls at age 6); Final Fantasy VII, VIII, and IX; Xenogears; Oblivion.

Oblivion is probably the one I can actually track the hours in and I know there's at least 100 between the two characters I played the longest. It made me sad that I didn't get into Skyrim nearly as much (~60 hours). I still want to do Morrowind since everyone says it's even better.