A few quick ones because i'm too lazy to go into detail: (SPOILERS!)
Zelda OOT:
The first time you watch the sunrise on Hyrule field. Being a fearful slave to the day/night cycle and then learning the song to manipulate it. Yay, no more scary dog-goblins!
Deus Ex:
When you finally escape from New York and land in Japan. Suddenly you realise that the world you thought encompassed the game was merely a stepping stone. Awe-inspiring. Generally, Deus totally blew my mind. It's fair to say that it started merging with my real life at one point. I didn't just care about the game, at points I literally felt like I was IN the game. Scary, huh? Shows how powerful computer games can be at times.
Baldurs Gate:
Everything about this game is epic, a true masterpiece. If I had to pick out one moment that defined the experience, it would probably be when I finally found the only hooded cloak in the game, and suddenly looked like Jesus Christ-with-super-innate-wizard-firepower. I walked around killing sheep for hours.
Tribes 2:
Another game that has hundreds of "mini-moments". The first time you look up and see a ship sweeping overhead with 4 soldiers parachuting into your base. The first time you join a 64-player server. The first time you sucessfully coordinate an attack involving your entire team. It's all special stuff.
Unreal Tournament:
The first competitive online FPS I played; it was just so slick! Racing around with that award-winning soundtrack pumping in the background as you tried desperately to get their flag back to your base. The soundtrack alone inspires frequent trips of nostalgia-fueled comatose whenever I hear it. It was also the first game that I joined a clan for, which really took it to a wholenew level. For the record, the clan was called [GBL]... "Gorilla bashing lesbians"
Ultimate Online:
The fact that I had never played an MMORPG before (and the fact that I didnt really understand how it all worked ) meant that UO felt truly awe-inspiring for months. One particularly special moment was when a GM wizard teleported me around the world, taking me on tours through some of the most dangerous areas. I remember feeling untold joy and exhiliration as this enigmatic, cloaked figure showed me dragons and goblins, occasionally thrilling me with his powers to tame anything we came across. I mean, can you imagine that frommy perspective? It was truly like discovering a land where you could live in a Robert Jordan novel.
Oh, and the time when I first visted a player-made village was pretty damn impressive. Complete with players blacksmithing, friendly duels and horse races.
City of Heroes:
The first time I got the "Fly" power, and realized that it wasn't just a gimmick, you actually could fly anywhere! Looking to the top of the highest building and the being able to swoop up there was great fun. Wonferully implemented.
Anarchy Online:
The first time I left the tutorial section and entered the city. Amazing. My first 3D MMORPG, it's just a shamethat my computer couldn't handle it. Still, the huge city still looked impressive at 2FPS.
Another magic moment in AO: Running back to the city with a friend after completing a 2 hour mission. we were both on critical health, wondering if we were going to make it, when suddenly out of the fog beamed our beautiful city - backlit by the rising sun - and we knew we were safe. I dont think ive ever felt such relief in an MMORPG.
Boy, when I get'a typin'...