Journey. This game caught me completely off guard too. I wasn't expecting it and I knew nothing of the game prior. I was just bored one summer day and decided to download it. It's kinda hard to put into words of what I felt during my time with the game but it in short let's just say it was beautiful and umm... peaceful.
Uncharted 2. Beginning to end. The game easily lived up to the gigantic hype that I had built up for the game. That in itself was an amazing feat.
Killzone 2. I forgot what stage but I remember stopping in the middle of a gunfight just to take in the amount of awesomeness that I was seeing on screen. It was before a bridge level or something.
Gears of War. I played this with a friend in SD my first time but holy shit was I completely floored. The presentation of the game just screamed next gen. Truly the first of its kind.
SOCOM 2. Jumped on single player and thought it was an 'okay' game. But then, I figured I would try going online since I had a PS2 network adapter laying around doing nothing. After playing just one game, my body was in complete shock at what I was experiencing. The map was Enowapi and I was there in a group of 8, talking and collaborating with real people, with a bomb satchel on my back, infiltrating a terrorist fortress atop a damn mountain. I've never really played an online game let alone a shooter before so it was pretty hard to grasp all that pure gaming goodness that I was witnessing all at once. Calling this moment amazing would be an understatement. It was mind blowing.