For foreign affairs, I would say diplomatic representation because communication with other countries is important due to the fact that it will keep us out of wars, make sure we are properly represented, and even affect other areas of this type of affair such as trade (because communication is heavily used). As for domestic affairs, I believe that healthcare is the most essential because people need to stay healthy to keep this nation running, reduce the chance of premature death, and regulate disease.
This isn’t an essay, but it could serve as your thesis or guideline; just add more to describe the points listed. This is also merely my opinion