I think the best sushi is at Kimonos at the Swan. The California Grill has some really unique rolls, but Kimonos has the best variety and quality ingredients.
I suspect Il Mulino is the best Italian on property. I'll be trying it for the first time next month so I can't confirm that yet. I have, however, dined at all the other Italian establishments in Disney, and I think the only one of those worth bothering with is Portobello at Downtown Disney.