I played lot of adventure games, but I think I never got so enthralled. The environment is so believable, so fascinating, I can't stop playing just to see what I can find behind the next door. And I've never seen before characters moving so smoothly, water effects so pleasant, heights so frightening.
Of course the brightest idea is the girl you have to take care of, that add a complete new dimension to puzzle solving. I consider ICO one of the best games of all times.