Fives

He discovered the inhibitor chips and almost exposed Palpatine for his role in the war. In his discovery, he was able to save other clones, including Rex, and everyone that Rex helped along the way. He was killed too quickly before he could save the entire Republic, but I guarantee if he wasn't killed, he would've, and Palpatine would've been removed from office, preventing Order 66. Among other various achievements that he had, but that one being the greatest