I've had many memorable moments in gaming, mostly when I finish a long, rewarding game or when I go into my "zone" and beat a difficult boss.
But three things stand out from the rest.
3 - my brother and I used to duke it out in mortal kombat 3 on the snes. We were pretty even matched so we decided we would settle it once and for all by mirror match ups with every character. I managed to beat him in the end as well as with his favorite one: subzero. Felt good to have bragging rights from thereon.
2 - I used to be REALLY into spiral knights. After about a year of release they had an update in which they added wings as an accessory. After that update, that became sole purpose. Wings.
After playing for a year and a half and totalling 1400 hours, by selling items, saving and in the end borrowing from friends, I was finally able to buy them. By far one of my best gaming moments.
1 - however, I guess the destination is not as important as the journey to get there. Again in spiral knights, where coop and helping each other is king is my best moment.
Me and a friend I had from the very beginning were still noobs the endgame boss. A couple of guildmates asked us if we could help them as they had never been to the boss. (The boss is actually beaten many times but it is the last boss)
We didn't think we would make it but we worked together and in that run me and my friend went from noobs to pros and carrying the party up until the final death scene.
Oh yeah, forgot mention my avatars