Mikhail Bakunin is probably one of the most well-known and oft-cited classical anarchists. His work is the subject of much debate between competing tendencies of anarchist thought and practice. He is probably most known for his collectivist variant of anarchism, his role in the IWMA, and his critique of the dictatorship of the proletariat. He is perhaps the most well known critic of Marxist tendencies. To read a large collection of his work in a clean format visit http://theanarchistlibrary.org/authors/Michail_Bakunin.html
One of the most interesting questions that seems to divide people today, in my opinion, is on Bakunin's understanding of power and human nature. How sold was Bakunin to this notion of a 'brotherhood of man' (as a founding myth, corrupted by the state) and how sold was he to his notion of destruction as a creative force? Perhaps there are "many" Bakunin's, like there are "many" Marx's.