Depends what you are after really, all MMO's have similarities and some have very stark differences. It's always been that WoW vs RuneScape fight. Personally, I hate WoW. The story was janky and all over the place and it made no sense what so ever and I could never get on with the way the quests are done or the way that the game played. I tried and it was just a no from me.
Now switch to one of my favourite games of all time, RuneScape. I love everything about this game. For me, I am a lore hunter and a lover of a hell of a good story. The story in this is told slowly over time following quests that are split into different catergories depending on what you want to do in the game and you are free to do whatever you want to do. The quests aren't "fetch this" and "kill that". The longest quest I had to do was over 4 hours long and it was so worth it, the story was well done and it all made sense and I actually knew what was going on! Another one took about 5-10 minutes. They all end up becoming intertwined near the end as well which I really like.
That is my piece on it though, I find it tough to explain my favourite games, for sure.
But one thing you need to remember is what you are looking for in it. Is it the world? The setting? The lore, the combat, all of that. So many variables