Episode V: The Empire Strikes Back. Though my favorite scenes are with Yoda, which conclude in Return of the Jedi.
Return of the Jedi kicks ass. I love the part when they kill Jabba the Hut: Leia chokes him with the chains! I also love how it ends.
Episode V was one of my favorite movies as a kid, and it's still one of the best movies I've seen from the 80's.
This scene blew my mind when I was a kid. It's still likely the best space battle I've seen, even though explosions in space shouldn't be "fiery" it still looks neat.
Return of the Jedi is without question my favorite Star Wars film, and it is the only one of the series that I would put on a list of my favorite movies. I feel the sense of fun and adventure was heightened with this one, I love all the Jim Henson creations and the action scenes, the lightsaber duels felt more dynamic, and of course I love how the Vader story concludes.
No votes for the Star Wars Holiday Special? In all serious though - I voted for the Empire Strikes Back.