definitely a toss up between Civ2, MarioKart64 and CS:S
Civ2 is probably the game i play the longest session on, if i dont finish a game of it even tho i save it i always start a new one the next time...lol
Mariokart64 is probably the game i've been most dedicated to beating records on and stayed with a long time and gone back to after taking occasional breaks. (18.something seconds on Wario Stadium....using the straight left over the wall at start line, but power sliding straight back over the wall behind the finish line, only ever managed it all 3 laps mebbe 3 times under 20 seconds, but what an achievement, most times i had to redo one of the jumps so took around 25-28 seconds.....hmm..wonder if i still got the score on a memory card, might have to whip out me N64 and check...heh...neone else ever manage that sort of time?)
CS:S is probably the game i've spent most hours on in the time i've owned it, but i can take breaks from it when i feel like it, Civ2 only had pee breaks, mebbe food...CS:S i stop to watch a TV show or movie.....