The Battle of Hoth all the way.
I'd say the Battle of Yavin at the end of ANH is a close second.
But man, everything about Hoth, from the actual location, to the prep before the battle, the walkers, the way Luke and the rebel's assault is intercut with Han and the rest trying to escape. Then to top it all of you end with Darth Vader himself showing up and being menacing.
Simply a perfect scene. Thinking about it, I can't recall any movie that has done a battle this effectively this early in the story.