Julius Randle I’d say.
You could also pick Jerami Grant although I would say his is more of a case of a bigger role/opportunity rather than his improvement as a player.
Forgot about Julius.
Thing about Don is IMO he's been overachieving since got in the League. I feel like he cant be overrated because he never was supposed to be as good as he is now. People forget that the Jazz were bout to blow that team up when they drafted Don, he wasnt seen as Gordons heir apparent.
As far as this year, Yea he's not closing games like he did in the bubble but the Jazz are still the top ranked team in the West, so he's doing something right.