If we are talking favourite MMO ever (rather than currently), then I am going to have to say UO , pre-trammel. Better than any other MMO I ever played by a loooooooooooooooooooooooooooooooooooooooooooong way. A level of freedom in an MMO that imo has been unmatched ever since, even now 12 years later MMOs are still lacking many of the features that UO had 12 years ago.
An MMO where you truly were part of a breathing world, with an unprecedented level of freedom. Want to have a character who never needs to fight anything, no problem you could do that. Want to tame a dragon and have it as a pet to fight for you, no problem you could do that. Want to steal items from peoples backpacks, no problem you could do that. Want to murder another players character, cut their body up, cook their flesh and eat it, no problem you could do that. Want to run your own player run town, with a tavern that is open to the public and manned by proper tavern "staff", no problem you could do that. Want to run a private shop in a building ingame, with npc staff working for you in the shop selling your items while you are offline to passers by ingame, no problem you could do that. Want to own your own ship and sail the seas, fishing up treasure maps which you then "solve" and go dig up the treasure and fight the defending creatures, no problem you could do that. Want to dye not only your clothing, but also your furniture and crafted items, no problem you could do that. Want to write stories in ingame books, that other players can open up and read, no problem you could do that.
The gameplay options were more variable and free than any MMO then or since has ever come close to.