I've played Everquest, Everquest II, WoW, and FFXI.
Of all those I think FFXI is by far the best. It's actually difficult, well balanced, I love the job/subjob system, the crafting is really good, and it actually encourages teamwork and interacting with other people (you know, the reason you play MMO's).
I just picked up WoW for the third time this week and the game isn't bad, I just hate how little I'm encouraged to interact with other people. Sure, if you do a raid you need a large amount of people and some quests require pickup groups, but for the most part I feel like I'm still just playing a single player game.
Except for the first 10 or so levels, I never felt like that in FFXI.