Even though I said BG2, I'd have to change my vote to Tetris.
There is no other game I've spent as much time on than Tetris. There is no other game in which I am as good as Tetris. This game was my best friend for a good 10 years of my life, and I still take it with me on every vacation. I'm not a person to often bloat about his skills, but I honestly kick some serious ass at Tetris. Most triumphant moment was when I finally found out there is no lvl 21 (by scoring 10 lines on lvl 20, normally the level would increase every 10 lines but starting from 20 the dropping speed remains the same, so I discovered). My next challenge is to find out what happens after you hit 999.999 points, as the score board only has 6 figures. I've managed to get to 270.000, which is pretty freakin' mad by itself.