definitely gonna have to go with

1) eve of extinction. im sure there are hardly any people who know what this game is, but I had it on the ps2 as a kid and played the shit out of it, could never get past the 3rd level, but it gave me a love for video games, and holds a special place in my heart
2) skate 3. its skate 3, just an insanely fun game
3) clannad. sounds pretty cliche but kind of changed my life. its a visual novel, not a game, but it's sold on steam as a game so I'm just gonna have to list it