There have been so many over the years but the one that sticks out for me on the PC is Ahserons Call (I've said this in my posts over the years).
You have to remember, at that time, this was one of the first MMO's - there was Everquest and UO around that time but this was the first where you had proper stats (you upped your run and you ran faster, etc.).
I remember wandering up town and the people running around were actual people! It sounds stupid now but this was years before WoW / etc. There were people talking, interacting with each other, learning new spells together and generally just having fun. It was a time when we ran around the world map just to see what was there.
I had great times on that game. Far to many to list here.
M.