So it's Kaizer, Destoroyah, and Hedorah. This is when it gets tricky. I think Destoroyah would cancel out Keiser, while Keizer would get Hedorah. Hedorah never seemed that smart when compared to the other two, or as vicious. I find it funny that in the German release of Godzilla vs. Hedorah, it was said he may have been created by Dr. Frakenstein.
Lol. Pretty much all of the German versions of Toho monster films have some kind of connection with Frankenstein, and is usually mentioned in the title along with the dialogue.
Again, I don't see what Keiser Ghidorah would do to Hedorah. He doesn't have the tools to take him down, and that's what it always comes down to in these (insert monster) vs. Hedorah match ups.
I don't think your claim of Hedorah being "not as smart" or "vicious" really holds any water. Your talking about three different monsters from three different eras of Godzilla, from three different films helmed by three different directors.
He was smart enough to flee from Godzilla when his life was seriously in danger, and even attempted to drown Godzilla at one point. He also destroyed all the JSDF helicopters that dropped oxygen bombs in his vicinity. I'd say he's at least fairly intelligent, if not more so.
Hedorah kills most living things by simply flying over them while releasing sulfuric acid. He doesn't need to be "vicious" when he literally kills things will little to no effort.