Phantasy Star Online is a good answer. I'm glad there are unofficial servers but it's not the same in terms of player count.
Since there's still some way to play PSO, though, my answer is Playstation Home for the PS3. I had a lot of fun on that thing and it was free if you had a PS3. I have a Quest headset but have no interest in the "metaverse" (really just a name for the game Horizon Worlds).
Currently playing: Skyrim, Hearthstone
Training log: https://powpowpunishment.blogspot.com