What is the category of businesses with highest star rating?
category refers to category_name; highest star rating refers to max(stars)
SELECT T3.category_name FROM Business AS T1 INNER JOIN Business_Categories AS T2 ON T1.business_id = T2.business_id INNER JOIN Categories AS T3 ON T2.category_id = T3.category_id ORDER BY T1.stars DESC LIMIT 1