For me, there are two. That's as best as I can narrow it down.
Final Fantasy VI -- Starting this game up on my SNES was the most important gaming moment of my life, without doubt. Prior to that, in all honesty, gaming was nothing more than a way to kill some time. I'd enjoyed my NES a lot, but the SNES just felt like more of the same. Platformers, beat'em ups, rinse and repeat. Other than Zelda, I was drifting away from gaming and getting into reading more than ever before. Then I happened to see FFVI at a used game store that I was only in because of my friend. I bought it on impulse and it blew me away. Depth in video-games, a fantasy world with multiple, well-developed characters, with love and betrayal and politics and an actual story. This is the game that made gaming my hobby, that got me online looking for other RPGs, that made me stumble across anime and Asian films and television. This is the game that directly led me to the PS1 because of FFVII, that prompted me to pick up Suikoden and Final Fantasy Tactics.
I owe a lot to that little gray cartridge with a Moogle on the front.
ICO -- I compare this game to Serial Experiments Lain a lot. Both of them kicked down walls in my head and showed me the vast potential their respective mediums actually encompass. I bought ICO when it came out, I don't even remember why. There was no internet hype back then, no expectations, so when it hit me, it came totally out of left field. I remember clearly the bridge sequence and how I just sat there, stunned, with the controller loose in my hands. It wasn't just a videogame, it wasn't just something I was playing, it was a personal experience where I absolutely and completely hated the antagonist, where I wasn't just trying to finish a game, I was trying to save a tragic young girl from a fate worse than death. I still hold it up, unconsciously, to other games, to see if they've been able to live up to something so simple, and yet so beautiful. For me, ICO kind of changed everything relative to gaming, most of all myself.
