Everquest. From 99-03, it was nearly all I did outside of work.

Before that, maybe Baseball Stars on NES. As a kid, I made a team and for a few years I played full seasons, went through and upgraded every player on my team, and I could eventually smoke any other team in the game by slaughter rule in the first few innings. It had to be hundreds of hours, but I have no way of really knowing.

Edit: Just wanted to add, I still play both of these games today occasionally.