I enjoy the odd computer game, but a few stand out for me as being truly exceptional. I wasted many, many hours playing these when I was at school.
Out of all the games I played, I spent the most time with doom. Doom introduced me to the world of truency, as well as game-modification and a few concepts that form the foundations of programming. Visit doom for more info.
UFO: Enemy Unknown
UFO: Enemy Unknown (aka XCOM: UFO Defense) is a superb game, that I think would really benefit from a community like doom's. How I crave for a modern source port / clone!