He was well known all over the world and even critics thought highly of him.

thought 是动词 think 的过去式，意思是"认为、思考"。 highly 是副词，由形容词 high（高的）加上后缀 -ly 构成，表示"高度地、非常"。 of 是介词，在这里与 think 搭配使用，表示"关于"。 thought highly of 是一个固定短语，意思是"对某人或某事物给予高度评价"。