Which of the Axis nations did the Joint Chiefs of Staff believe should be the primary target of the Allied war effort?

According to all the movies made on the subject including documentaries, the war in the Pacific had less attention paid to it than the European (read German) theater.

And yet for America, the fighting in the Pacific was probably much more brutal and primitive than Europe.

But that can't stated categorically. Europe was mainly a conflict between the Soviet Union and Germany. Some of the worst battles (for both sides) were fought on Russian Soil. You could look up Stalingrad, Leningrad and Kursk. These three battles were among the bloodiest (if not the bloodiest) in all recorded history.

So I guess it depends on which historian you ask.

You likely will do all right if you answer Germany, but it was not nearly that simple.