For me it will always be Everquest, the game that popularized MMORPG's. Yeah it's old and the graphics aren't as pretty as newer games, but the gameplay, the community, everything was great. I haven't found another online RPG that compares, WoW is (IMO) a very dumbed down, care-bear game in comparison. I've played other RPGs but none I liked as much as EQ.
Of course, most of us remember our first fuck very fondly too...