1. The end of Deus Ex.
Overall, I kinda liked the game. The cyberpunk aesthetics were great, I liked how it combined first person shooting with RPG elements, and the story didn't disappoint. But there was a huge generation gap, and the bugs, the shitty graphics, and the primitive-feeling gameplay made it fall short of impressing me beyond saying "Oh, that must have been pretty good for its time." The real moment I fell in love with Deus Ex was the moment I was given the choice between 3 endings, and I didn't know which one was right.
Although I guess you can say my heart was struck when Denton said "You mechs may have copper wiring to reroute your fear of pain, but I've got nerves of steel."
2. First seeing Megaton in Fallout 3.
From the moment I opened my eyes and heard the warm, loving voice of Liam Neeson, I knew I was going to like this game. When I escaped from the Vault, it further solidified in my mind that this was the frontier I always wanted. Megaton was essentially the realization of that fantasy. When the huge steel doors opened in front of me, I was amazed at the huge structures made out of shitty scrap metal, the futuristic technology combined with laughably outdated styles, and the random-ass nuke in the center of town. The civilized world is boring and unremarkable. As Yahtzee said, the purpose of exploration is to make the world more boring. So seeing the world once again unknown and worth discovering (though with the most frightening means) absolutely fascinated me.
3. The telephone in No More Heroes.
I guess this is a bit of a cop-out, because everything in No More Heroes captivated me at first. Everything was just so...gratifying. The lightsaber moves, the wrestling throws, the otaku taking down assassins with giant swords. It was an amazing experience. The one that I expected the least was the telephone. Such a shame and Xbox and PS3 gamers will never be able to experience this little bit of comic delight.
4. Falling down a building in Mirror's Edge.
First game to actually make me afraid of falling.
5. The very first level of I Wanna Be the Guy
It was amazing. It was unexpected. It was brilliant.