Which kaiju is considered Godzilla’s biggest rival?

King Ghidorah is that one friend who always starts drama at parties, except the party is planet Earth and the drama involves leveling entire cities. This golden three-headed space dragon showed up in 1964 and immediately decided Godzilla needed to be taken down several pegs. While most monsters terrorize one city at a time like civilized kaiju, Ghidorah flies around shooting lightning from all three mouths like he's got somewhere important to be. The guy is so consistently awful that aliens keep hiring him as their go-to Earth destroyer. What seals their rivalry is that Godzilla usually needs backup when Ghidorah comes knocking. Even when Toho tried making Ghidorah a good guy in 2001, they had to make Godzilla possessed by angry war ghosts because regular Godzilla wasn't threatening enough to justify their golden boy switching sides.