Final Fantasy XI. It's been online for over 20 years and still has a devoted player base. The game's scale is so epic that many people still haven't beaten the expansions.
When it came out it was so far beyond what we had seen in an MMO before - The only competitors were UO and Everquest, but the graphics, music, complexity, and storyline were miles beyond those games.
It's a game with unimaginable depth of play that takes years to master - not like the hand-holdy easy games we get nowadays. Truly a gem the likes of which we will never see again.