For me it's a tie between Empire and Return of the Jedi, I guess if I had to choose I'd go Return of the Jedi tho.

The final battle between Luke (with the green saber) and Vader is in my opinion the best lightsaber battle in the entire series. I hope that's what Episode 7 tries to replicate. That battle wasn't ridiculously slow like the ones in A New Hope but it also wasn't like the crazy over the top Quadruple backflip style of the prequels.

It was just two very skilled swordsman fighting, perfect.