ICO is every bit as good as Shadow of the Colossus, and you owe it to yourself to play it.
It's got clever puzzles, a touching story, and incredibly memorable setpieces. Like Ueda's other works, it's not too long, and learning how to speedrun the game was a fun time too!
You can't play this game on any modern platforms, but the PS2 version is easy to find used and runs well in emulators.