The fifth movie and it introduces the best antagonist Godzilla ever had, King Ghidorah: a golden three-headed dragon from space(According to this movie).
After destroying civilization in the planet Venus, King Ghidorah sets his sights on Earth. Upon arriving, he begins his destruction on Japan only to be stop by one of Mothra's offspring. Unable to stop him, she seeks the aid of Godzilla and Rodan, a monster resembling a pterodactyl, to help her defeat the alien monster.
This movie is another great entry in the series but it also starts to dumb down Godzilla. From this movie on to the 70s, Godzilla becomes more of a good monster to protect Japan. Some scenes here a a little too cheesy, specifically the scene where Mothra, Rodan, and Godzilla have a discussion about defeating Ghidorah. But overall, it doesn't ruin the fun of the movie.
Ghidorah is a fun to watch and the three on one fight is a great spectacle to watch. Recommended.