Ico was awesome. I love how it told a story without words or narration. That's pretty impressive.
Edit: Thinking back, I remember Ico more fondly. Shadow of the Colossus was impressive to me for its unconventional design, like Ico, and the battles with the colossi were incredible but in between those battles there wasn't much fun to be had. Sure, there was the wonder of the world and a slight element of exploration, but it was kind of boring. Ico had far fewer lulls and when it did, there was still a large sense of weight to the game.