ico
ICO is every bit as good as Shadow of the Colossus and still stands out today.
The game is full of clever puzzles, touching character interactions, and incredibly memorable setpieces. I'd highly recommend checking out MystICOde's Web site and YouTube channel to learn just how deep the game mechanics and interconnected environment design go.
Like Fumito Ueda's other works, ICO is short and sweet, and learning how to speedrun the game was a fun time too! SanchoPanda's video tutorials were a huge help here.
You can't play ICO on any modern platforms, but the PS2 version is easy to find used and runs well in emulators.