Geez, there are so many... it would be really tough for me to pick just one.
Some of the most memorable moments for me were:
Pac-Man, Atari 2600: The programmers were so cheap, they didn't even bother to put in separate sprites for when you moved up or down. When you moved down, it looked like Pac-Man was shoving the dots up his... <.<; I was like 6 or 7 years old, and I remember thinking that
I could have done a better job.
Lode Runner, Commodore 64: All I had seen before this point was Atari 2600. I remember thinking how amazing the animation and sound effects were.
Metroid, NES: Finding out Samus Aran was a woman for the first time and thinking how fucking cool that was.
Phantasy Star II, Sega Genesis: The sad, sad moment when Nei, the original HUnewearl, dies.
Rock'n'Roll Racing, SNES: George Thoroughgood & The Destroyers "Bad To The Bone" as the title song... 'nuff said.
Doom II, PC: Playing this game in my dorm room in the dark with my roommate watching. He and I both get the absolute shit startled out of us as I round a corner and bump into a Cacodeamon.
Final Fantasy VII, Sony Playstation: Do I even have to say anything? This game rocked my world.
Phantasy Star Online, Sega Dreamcast: I come home one day and ask my friend and roommate, "Hey, what's that you're playing?" He lets me try it out... I make a HUnewearl and name her Layrinn... five seconds later, I'm hooked for life. The rest, as they say, is history. ^.~