There's only two MMO's I like, but I like them a lot, and they are both practically OG's of the genre. City of Heroes and Everquest. And I like these two because you can play them as single player RPG's with large and rather open worlds.
Right now I am playing City of Heroes. It has a summoner class called the Mastermind that is the best implementation of the class in any MMO, any RPG, or really just any game. You get about 6-10 allies, depending on your build, and your powers can come from a vast array of sources. You can run an army of mercenaries, be a necromancer who controls the undead, make dark pacts with demons, run a clan of ninjas, even be a druid and create magical pacts with animals, and more.
I chose to run a clan of ninjas and begin my journey on Praetoria, an alternate version of the original game world where all of the most famous heroes gave into their darker sides and ran the world with an iron fist. Good and evil isn't defined by whether you are a Hero or Villain but by the choices you make as either a loyalist to the Cole regime or as a member of the resistance. You can even be a double agent and work for one side while reporting into the other and sabotaging one sides goals.
Nowadays I dual-box MMO's when i'm not running group content, as I find difficult MMO's are a lot more enjoyable when you are building two characters as mutual support. You won't be defeating Archvillains or doing the best quest-lines solo without having an end-game character (though any build can do it if you're good), so this will speed me along that path.
My second character is a katana-wielding stalker who has ninjitsu as a secondary power, so he is a perfect fit as another member of the clan. I'll send him in for his massive backstab damage, send the rest of the clan in who are controlled by the Mastermind, while he runs out and goes back into stealth. Rinse and repeat. Very efficient, though the team struggles somewhat against flying enemies. Less so now that I have an Oni who is controlled by the ninja mastermind, who has magic to take them down.