Depends what you want from an mmo.
Do you want a casual experience where you can come back after a few months of not playing and still be relevant in new, singleplayer story based content? Where events are completed on maps and everyone participates? Where fashion is the most important aspect of the game? Then play Gw2.
Do you want a more structured and hardcore raid progression experience requiring lots of time investment with abysmally boring combat but fun pvp and an active community that socialises a lot? Play WoW Classic.
Do you want to be a filthy grinder who will never be able to catch up to the same level as the no life veterans and they will always have farming spot preference because they one shot you? With an amazing node system that makes gathering/harvesting somewhat interesting. Play BDO.
Do you want a modern mmo experience with absolutely amazing raid combat yet a community who chooses to not really talk all that often compared to other mmo's, unbalanced pvp and millions of players? Play modern WoW.
Do you want progression to be the forefront of of the experience, where some tasks even have you grind out for dozens of hours to get one specific item that makes grinding the next task like 5% more efficient? You will never max your character/account but you will fall in love with the stories and the world surrounding it's unique quest system? Combat is horrible and turn based but as you progress, becomes more about timing, game knowledge and the clicking skills of a starcraft 2 player to be able to even stand a chance in pvp? Play Oldschool Runescape.
Do you want raiding to be the sole point of the game, where pvp is so bad, no one bothers with it. With raid progression and combat more focused on statistics and animation locked abilities, rather than fluidity? Play FFXIV.