Hard to say... I voted for the Death Star I battle mainly because of the great attention to detail that was given to the effects. Unlike in ROTJ, the ships don't simply explode into a fireball when hit, they each go down in their own individual way. My favorite would have to be Gold 5(?), whose right engine spins off when hit before he goes down in flames.

That having been said, the Battle of Hoth is an amazing technical achievement, and the Death Star II battle is amazingly simple to follow considering it uses all three axis' and multiple ships (unlike say, Independence Day, in which it's very confusing to tell who is who).

I don't have much to say for the Episode II and III battles but the lightsaber dual in Episode I had some great stuntwork and coreography.