A very recent one for me was beating Child of Eden and experiencing that ending and credit sequence for the first time. That was probably the most emotionally moving thing Ive ever experienced in a game.
As far as gaming related some of the LAN parties my high school computer teacher held in the computer lab were incredible. The Halo 2 launch was also an extremely good time.
I suppose you could never explain to the most ingenious mollusk that such a creature as a whale existed-Emerson