Return of the Jedi is my favorite, but that doesn't say much since I love them all.
Return of The Jedi is basically my favorite because of the Luke and Vader duel. It is one of the most emotional scenes out of all of the Star Wars films, and the saber fighting is just awesome. It is my favorite saber fight by far. The OT duels went by too fast IMO.
Also, the Battle of Endor was flat out fantastic. I'm truly amazed Lucas was able to pull off something so dificult back then.