I think I have stories from all my games. Utopia - plotting against the 4-500 people strong enemy alliance by building a network of 2-300 loosely connect people with various interests, balancing various alliances and allegiances, trading together a team of 25 dedicated team mates, racing for the top but being pulled down by trying to manipulate one alliance too many. And doing it again next 2 month round.

EQ - pulling all PoF bosses without touching any trash mobs, swimming through lava with a one hit kill boss behind me to save time, spending 3-4 hours with 70 other people to clear council of rathe, progressing from killing dragons in the ice zone to clearing plane of time, aoe kiting an entire zone... (Eq bard is still the best designed class I saw in any game)

UO - getting an order for 200 suits of coloured ore, working to complete it like mad for 2 weeks and being robbed by a bug in the new stealth feature just before completing the sale... Hitting GM smithing, buying a tower with 3 friends, exploiting blade spirits for resists forever... First time buying a house (just north of Minoc)

QWCTF - first time winning a clan game by timing quads perfectly on e1m2 for a full map (I could still run that map in my sleep), taking on and beating a clan miles better than us by playing from the same room, playing a match entirely with the boom stick in a clan game and still typing kills, the feeling of flying into a base with perfect grapple moves despite two guards with ro and str/res runes, the feeling of bouncing along on the long stretches of e4m3 with floor grapple speed jumps...

All of that more than 10 years ago and still vivid memories for me.