User blog comment:Tacozilla99/What is the strongest Ghidorah incarnation from 1964 to 2004/@comment-1060835-20160213232011/@comment-25285745-20160214041647

Keizer Ghidorah was confirmed by Toho to canonically be the most powerful monster in the Godzilla franchise. Now, the Rebirth of Mothra III King Ghidorah could be considered outside of the Godzilla franchise so he could be more powerful.

Also, Keizer Ghidorah can fly, he just never does it in the film. He does showcase the ability in Godzilla: Ongoing.