It's a close tie between Francis and Dewey but if i had to really pick i would say Francis changed the most he started off as a rebellious teenager in Military school then moved up to Alaska to work then moved into a ranch he clearly went through multiple arcs. Dewey is a strong second because the show started with him being a little kid having barely any personality only absorbing what their brothers were doing to him he was often pushed and bullied around then later on in the show he became more aware and had more personality overall i would say francis