IMO Final Fantasy XI is hardest MMORPG because you die easily when solo with anything above your level. Even few easy prey monsters can own you.
A lot of quests and whatnot requires very strong team or with few skilled classes that know what they're doing. Getting to level 75 takes a long time, unless you do exp grinding non-stop which could take few days to hit 75.
There's one of hardest bosses that isn't defeated yet. That boss is Pandemonium Warden. And there's other one that is also considered very tough and it has been reported it's been defeated by few linkshells, is Absolute Virtue. Both takes more than 18 hours to defeat them. Wow.
Personally, I like Final Fantasy XI far more than WoW or any other mmorpgs. It's tough, has a decent story, a lot of team works, and it's fun. I'm not much fan of PvP myself but FFXI has it (I never tried PvP so I don't know how good it is). Only bad things about FFXI are, it's also damn long and requires a lot of work to get all sweet end game, rare items as well get yourself in respectable linkshell so you can get join to fight against all kind bosses or notorious monsters.