Best Sheraton in the world would depend heavily on how to weigh the different aspects (location, service, room, Platinum treatment, etc.) because all Sheratons have some weakness. So all I can say is that these Sheratons get some votes from me, for one aspect or another and that overall they are quite good:

Sheraton Pran Buri Villas
Sheraton Grand Taipei
Sheraton Macau
Sheraton Shanghai Waigaoqiao

"Best in the world" would probably also limit the conversation to Asia because hotels are better all around over there. It is not realistic to expect a Sheraton in North America to compete with a Sheraton in Asia.

If we consider local market characteristics, then I would add to the list Sheraton Zürich. And if I need to name one in North America, then I would say Sheraton Saint-Hyacinthe.