King Andrias by The Core and his father.
This is not important, the question is who'd be a planetary threat, as far as we know, Ghidorah can't end the world by themselves, they needed the title of alpha to order all Titans in the planet to do so and Skar King needed a specific Titan for that.
Technically both, King Ghidorah managed to take control of all Titans and ordered them to destroy the world (i'm not sure if they could do such thing on their own) and Skar King managed to take control of Shimo, who can start a new ice age whenever she wants (which OBVIOUSLY Skar King can't do on his own).
Pink Diamond/Rose Quartz
The black king! The amazing being capable of making things black! And that's it