It is the left one. In the big fight Godzilla ripped off the left head. You saw it grow back when Ghidorah was on the mountain. In the post credits scene the guys who sold the head pointed out they had no other way to make money because the oxygen destroyer had ruined in the fishing.

I find the idea of the left head growing into a new Ghidorah who is more whimsical to be a very amusing idea.

I lol'd