Complex sentence: Anti-revisionists consider themselves the ultimate leftists on a spectrum from communism on the left to imperialist capitalism on the right, but Stalinism is often labeled rightist within the communist spectrum and left communism leftist.
Split into simpler sentences: Anti-revisionists consider themselves the ultimate leftists on a spectrum from communism on the left to imperialist capitalism on the right. But Stalinism is often labeled rightist within the communist spectrum and left communism leftist.