When I watch ROTJ, and in particular the speeder bike chase, I often go back with my remote just for hearing those sounds!
Some of the TPM Pod sounds are really awsome too. The race really worth the listening!
One other sound is really really cool in AOTC: Jango's charges in the asteroid field. The first time I've heard it, I was "...WOW!".
I also love the creatures sounds in the whole saga. Ben Burtt has really made a great job on that!