Mine would be EVE Online for many reasons but mostly for the roles you can have in this game. And by roles I don't mean tank, dps, support, etc.
During my tenure I have taken on many roles. A cannon fodder, a miner, a merc, a spy, an accountant, a leader, a jerk
and so on.
That is what made me feel like I was part of a virtual world. And this is what I expect from an MMO, otherwise I'd rather just play a singleplayer game.