User blog comment:Miskos3/Fairy Tail 521: The Strongest Wizard, Review/@comment-97.104.204.83-20170211092559/@comment-7418318-20170211102428

Laxus is easy: Zeref had his little pet-demon spying on the GMG and Laxus did quite a show there by defeating Jura on top of being quite famous to boot. And he was also the one to help FT escape from Ajeel over on the other continent, which was seen by August, who would be able to evaluate his strength quite well I'd imagine.

Jellal though... that is quite intriguing.