Most Improved might have to go to Dorian.
I know you can argue Ronnie didn't 'improve' from 2002 to 2003, but I would say that he had a gut in 2002 with small arms, so although his gut was back in 2003 his limbs and delts were bigger/wider than ever, thus improving his symmetry a lot. He certainly won more commandingly that year than the previous year, so at least in the eyes of the judges it was 'improvement', and 40lb is a big difference.