Just thinking of army commanders off the top of my head, not counting brigade and division commanders:

Best Union: Samuel Curtis...doing an about face with his entrenched army and defeating the Confederates followed by his march to Helena.

Worst Union: Nathaniel Banks

Best Confederate: Richard Taylor maybe,

Worst Confederate: Theopholus H. Holmes. I also don't understand the infatuation many have with Sterling Price. He never did anything as a commander.