OH YES, OH YES, I've been waiting for this question for so long
I don't tend to buy games as an afterthought, but that only makes mine all the more special:
Rome: Total War,
Played it, loved it, forgot about it for a couple years, bought it cause I was bored, and 200 hours of only campaign gameplay later, I'm still addicted
It's also the game I thank for getting me into RTS's in the first place, and now RTS's make up at least 60% of my game time, WOW did Rome change my life and interpretation of video games as a whole.