Both are likely equally smart.
Zilla Jr. has the speed, and agility advantage and able to breathe Atomic Breath. However Hanna Barbera Godzilla is stronger being able to throw spacecraft's into space and Monster's his size to the moon. He also appears to have more abilities laser eye's and fire breath. I'll have to go with the Hanna Barbera Godzilla, but Zilla Jr. won't go down easily.
The King can kick the living Crap out that that...Dinosaur who called itself Godzilla.