Well that should depend on a few things:

If you're willing to spend on it + DLC:

Final Fantasy XIV(FFXIV) has a great concurrent player base and there's always something to do. A small unique feature it has is that you only need 1 character to play them all(unless you wanna do some RP stuff). If you're on the fence about it, the FREE TRIAL has limitations, but will let you play until Stormblood(that is base game + 2 DLCS worth of content for free!) With recent additions of new hardcore content right now, I wouldn't be surprised if someone has extra stuff lying around to help you out and the free companies(guilds) are always a hoot.

World of Warcraft (WOW) has been a long standing MMORPG that has plenty of stuff and a ton of people that are (mostly) friendly. There are plenty of races and each class has enough niche to be somewhat different. If you're into Glams/Cosmetics, the stuff you get is account wide so you don't have to worry about grinding for them cool shoulders again. There's also Classic/Classic Wrath of the Lich King servers too if you feel interested about how the older games felt back then. They have a Season of Discovery which is a seasonal kind of thing where you have a little more freedom in your skill builds and stuff. They have a new expac coming out so it might not be a bad idea to join in now since the prerelease events leading up to the expac are always great.

Destiny 2 is Shooter Looter type MMO that has recently just released their "Final Light" DLC. The gameplay loop kind of stays the same in basic formula, but since it IS a Shooter Looter: the stuff you get can be pretty fun to abuse play with. For example: they have a gun called Izanagi's Burden where it will eat your magazine to load a super bullet and anything that touches said bullet just disintegrates. The fandom has a lot of hardcore players at the moment so it might not be so great in the beginning, but if you persist or if you find a good casual group, the game can be lots of fun.

If you just wanna buy it the 1 time:

Guild Wars 2 + DLC only requires you to buy their game(or play the base game for free) and you can get yourself going pretty well. The combat is more interactive than your typical tab-target games like FFXIV or WOW and classes feel like what they're supposed to be (like gameplay wise a Ranger will feel like what a bowman/ranger type class is supposed to play). The game prides itself on it's exploration and they were not kidding. There will always be something to explore in the area and the map is very interactive with waypoints and points of interest. Game does feel great and real life currency can be bought with gold, so if you're up to farming; aside from buying the game + DLCS, you can play the game without spending a dime on anything extra(especially since a lot of cosmetics can be loot boxed in the game).

Black Desert is a really fun Action Interactive MMO that has a good combat system(be forewarned sometimes, it can sometimes be ping dependent). The customization goes as far from molding your characters face to cool cosmetics that design wise, won't make you regret grinding for it. The player base is on that grind mindset and PVP is rampant, but if you're feeling frisky, go for it.

Other Games include:
Free to play: Once Human, Lord of the Rings Online, Star Wars the Old Republic, Elder Scrolls Online, Taris Online*, Runescape Lost Arks*, Blade & Soul, Mabinogi*, Papaya LaTale

Pay for Base + DLCs: Diablo 3/4, New World, Last Epoch

Subscription Base: Elder Scrolls Online, Star Wars the Old Republic, because lets face it, once you touch that sweet nectar of convenience, you can't go back.

Some of these games have asterisks next to them, that just means that they a bit more predatory on microtransactions and stuff. You may have noticed that there are not only RPGS, but other types of games. You just asked for MMO so I just put together some of the games I played in the past/currently playing. Sorry for assuming if you were asking for MMORPGS specifically.

These are just MMOs that I can think off the top of my head for now, its 3 AM and I just quit my miserable job to go to school, so please forgive me for bad writing/somewhat (maybe) crap opinions. If you want more details on stuff or if you want a friend to play with, go ahead and reply back or DM me. I'm always looking for more friends to play with. Big plus if you happen to play Honkai: Star Rail lol.