Rush 2, I will never forget those opening credits, or the experience that followed it.
Halo CE came across it by accident in December of 2001 at my relatives place and didn't know about it, got me hooked on video games and I never looked back.
KOTOR 2, I thought it looked like a another generic crappy sequel, and it would have been if not for the way the story was woven together. It changed the way I thought about the world around me, the reasons people do the things that they do and action and it's consequences, intentional or not. The fact that it did so against my will, because i could not come up with a purely logical counterpoint without lying to myself about some of the more salient points Kreia brought up, made me understand how one might feel when they "fall" to the "darkside". Probably the most well thought out game I have ever played, good enough that I couldn't bring myself to care about being bothered with how unfinished it was. Funny too.