Godzilla's arch nemesis MechaGodzilla vs King Ghidorah

MechaG is a good candidate, but I'd still give the title to King Ghidorah.

He (and/or the Ghidorah species) and Godzilla have more of a history.

It's pretty close, there's no real clear winner it depends what you look at more. MechaGodzilla has a better record against Godzilla for the most part and even in the losses tends to put him more on the ropes. Ghidorah has been in more movies and the series overall longer and tends to take more than one monster to beat, even if in some of those instances it was complete slaughter when it happened.

I wanna go with Mechagodzilla if only for the purpose that in every instance he's had one goal, to kill Godzilla, Ghidorah was a spacemonster that came to earth to destroy it and Godzilla got in his way but he didn't come SpaceGodzilla esq to specifically to kill Godzilla. In every illiteration Mechagodzilla was made to fight and kill Godzilla that was his purpose. Of course it's still a toss up, but for that reason I slightly favor MechaGodzilla in the argument.