I'd say Revenge of the Sith is my favourite. Overall I enjoyed the prequels story line much better even thought Ep 1 wasn't very good. I think what makes the movie so great is the light sabre fight between Anakin and Obi-Wan. "You were meant to destroy the sith, not join them! To bring balance to the force not leave it in darkness! You were my brother Anakin! I loved you." "I HATE YOU!!" That scene there was my favourite in all the movies.
Empire is a close second though, many great parts in it. Solo and Leia was great. "stop that." "stop what?" "my hands are dirty." "So are mine what are you afraid of?"