He is widely thought of as one of the most promising young artists in the country.

thought 是动词 think 的过去分词形式，think 的意思是"认为、思考"。 of 是介词，表示"关于、属于"的关系。 as 是介词，表示"作为、当作"的意思。 thought of as 是一个固定短语，意思是"被认为是..."，表示被普遍认可的看法或评价。