I would say that technically, if you count Bravely Default as part of the FF series (albeit a spinoff), then Ouroboros would be more powerful than Exdeath or even Cloud of Darkness. Exdeath and Cloud of Darkness could erase parts of the universe to nothing, and more if given the time. Ouroboros did it. Ouroboros erased whole multiverses, and what's even more frightening, he did it casually and with ease, just to taunt the heroes. I'd say that puts him on a whole different level on the threat scale.