FF14 was the closest an MMO ever came to matching WoW for me, but it was missing some key stuff to it that would definitely prevent me from making it my main game 'forever'. For example even if I'm not a super active tryhard PVPer I feel like that's an important element for an MMO. Even caring about PVP at all in that game made me known as the oddball PVP guy on the server. Meanwhile the devs, who were allergic to even the concept of UI mods in PVE, cared so little they let people bot and cheat in PVP with no consequences. Of course I have no idea what they've done with it with the expansions but I can't imagine they did a complete 180 on it.
WoW lost its luster for me in early BC but I've come back for every expansion. Never for more than a few months, but I played Legion well into Mythic Nighthold before finally burning out. I still don't even think it's a bad game, so it wouldn't be hard to fall back into it. A lot of my friends still actively play it too. It's a safe answer, I guess.
I wonder how far we could stretch the definition of MMO for this, though. Many games that I'd normally categorize as 'diablo-likes' blur the lines by adopting many MMO conventions, like shared open world zones, raids etc. Meanwhile the few surviving traditional MMOs are very different in terms of server structure now. WoW's cross-server technology nowadays has resulted in a very, very different kind of game from the olden days of completely separate servers. Massively Multiplayer used to have a pretty easy definition but now we've got all kinds of live online games that share player interaction in some way.
