It's really hard to pick just one. My life has been entwined with games and game history, and there is so much nostalgia, so many classic games, it's hard to choose. I have a serious affection for a great many N64 games, as that was my return to console gaming after a college hiatus. And I played almost everything to 100% completeion - Mario64, Mario Kart, Golden Eye, Blast Corps (never got all the platinums), DKR, 1080, Wave Race, and on and on.
I can remember playing Pong, Asteroids, PacMan, Donkey Kong, and their ilk when they were first out too.
However, there has to be one crowning achievement.
Even in the early days, gamers loved to find glitches in games. Sometimes to your benefit - find the one spot that enemy bullets can never hit you - other times for silly fun - find a way to slip past the barriers at the side of a road and drive forever through homes and over lawns.
I rememeber one day I was on break at work - a movie theater, and I watched someone play a new game in the lobby- the sequel to a truly dull and uninspired platformer. But there wasn't much else to do for my 15 minutes so I watched, and saw as he found one of those loophole. He managed to jump his guy out of the playing area to run around where the score was kept. We laughed as the counter ticked down. So he started running, and running, and....
Shit.
Warp zone.
We got played. The game WANTED him to do this. Talk about a paradigm shift. Here was a game designer actually encouraging players to poke and prod their way through his little video game world.
Of course that game was Super Mario Bros.
And I have never played a game the same way since.