See riddles aren't supposed to be obvious or they would be no fun...
Thats why ragnarok is the best answer because you have to think out of the box...
Hey I can get the sword!
And if I use the esper, its a huge sword...what a difference!

Edit: Oh, and if you can only obtain it in WoR...then it can't be Shadow since you can hire him...Odin is not the same as Raiden, they have different everything...and Gogo, where's the choice mentioned in the riddle...not picking him up?

Double Edit: Answer... Mog?