For most improved I'm going to have to say it's gonna be Luis Scola. The guy was an absolute beast in the Olympics and outplayed Manu Ginobili, Fabricio Oberto, Andres Nocioni, and basically led Argentina to a medal.
He put up good numbers last season, and with Artest coming to Houston and giving them another option, his numbers could go down a bit.
He outscored Ginobili in Argentina and was basically Argentina's most dominant scoring threat. I think he can be more of an offensive force than Artest. Artest and Scola would have to battle for the 3rd option. Maybe sharing that position.