Mine has been FFXI, leveling my Black Mage (BLM) in valkrume dunes. The random people joining your party, the sheer fear of agroing the wrongs mobs, the music and the satisfaction of leveling up in such tough environment which took weeks. Reaching max level took months but you were truly an expert on the job.
I wish I could re-experience something like this but today's MMOs is all about the rush to the top.
I played EverQuest as a kid (when I was far to young to be playing MMOs) and it completely captured me, I have been trying to chase that itch for a long time, but modern MMOs just don't have the same feeling as they did back in the day. WildStar was the closest, and I loved WoW from vanilla->Wotlk, gave Rift a good run, but everything in like the last decade has just felt a bit different. I think because basically every game is an MMO these days, there is nothing special about MMORPGs. Plus the availability of the internet to give answers/information means that you have no reason to actually talk to other people in the world beyond socialization, rather than needing to interact to just get around.