I've also mostly enjoyed online multiplayer games, but I play a bit of everything. I think these are the games I've played most in order it must be:

World of WarCraft - Been at it since a year or so after it originally launched. Played a lot of Guild Wars 1 before that, and also lots of other MMO games before and in between, but WoW is the one I always keep coming back to. Longest break must've been like 6-7 years around the Cataclysm expansion, and then back at it at the end of Legion. It's on the back burner now again because of another mediocre expansion, but I still check it out occasionally. I think I must've sank at least 5000+ hours (probably way more) into this game over the past 20 years.

Elite Dangerous - Been backing it since Kickstarter, had high hoped for the game and it was fun while it lasted. But I lost all hope in Frontier in managing this game. They're only focusing heavily on microtransactions, currencies and paid early access content now. Must've been about 1500-2500 hours or something, but I've been out of this one for a couple of years now. I had high hopes for the future of this game, but Frontier is a master of promising glory and delivering disappointment.

Squad - Last, but certainly not least for me, around 1400 hours playtime (including the testing branch client). Got into this game right when it launched into early access on Steam. Was very tired of the themepark rides that Battlefield and CoD were turning into, but didn't want to commit to Arma's milsim style either. Squad fits in between perfectly. Also one of the greatest game communities I've got the pleasure to be playing with, never had so much fun with completely random strangers. The mandatory voice-chat really ties it together to create amazing and fun moments. This game really taught me that voice chat in games can actually be good, and not just kids spitting insults.

That said, I have a lot of games on multiple platforms, that I usually won't ever hit a 1000+ hour mark on most of them anyway. Like I played Witcher 3 twice (and a half I think). But that's like only 150 hours or so total. And this year's favorite has been Helldivers 2 by far, but that's only been almost 300 hours so far.