I haven't put an extensive amount of time into many MMOs; primarily only two: Runes of Magic and Guild Wars 2.
Runes of Magic: I got introduced to this game back in high school. It's basically a "WoW Clone" type of MMO, but with a genuinely cool dual-class system where you also got unique skills based off which combination of classes you picked. For example, I played as a Priest/Rogue for a while as a healer, which made all my casting times (for healing and damage spells) be shorter as well as getting several dark/shadow damage-over-time spells. I played it off-and-on for about 2 years until I had a fairly bad experience with another player in my guild and decided to overreact and make a completely new account on a different server. When I swapped servers, I had promptly found a new guild with lots of cool people and that's when I started to really get into MMOs. It was really fun doing things (be it quests, world bosses, dungeons/raids, etc.) with a decent-sized group of all friendly people who you see and communicate with on a regular basis. It was about 2-3 years after I joined this guild, that Guild Wars 2 released, and I found out that this guild originated from Guild Wars 1.
Guild Wars 2: On release, the levelling experience from 1-80 was actually pretty good and I legitimately enjoyed the crating system. However once I hit the level cap, there just wasn't much to do (yet). Additionally, the style of combat as well as the way you build/setup your character were completely different from what i was accustomed to. As a result, I had left the game for a few years and tried it again when their first expansion dropped. I still didn't find myself enjoying the game too much for the latter reasons listed above. Back in 2019 I gave it a shot again and you know what they say; third time's the lucky charm, or something? :p To this day I am still playing it, and absolutely love the combat now and can't look back.
Other Games: Every once in a while I have tried some other games, and have played them for maybe a few months, but no more. These include Runescape (I still recall endlessly mining in this game back when i was in elementary school), FF14 (I had some good times in ARR learning the game and dungeons alongside some of my friends, but man do I hate the combat pacing and GCDs), Rift and Tera (I don't remember much at all from either game).