For me its:
ANH: When obiwan makes his intro and pulls down his hood to say "hello there" to R2. Coupled with the harp chord by john williams, its one of my favs. Another is the cantina intro. and lastly, the x-wing, y-wing attack on the DS!
ESB: I love the transport evac of hoth, where the ion cannon fires, temporarily disabling a star destroyer, then its announced in the action packed hangar, "the first transport is away"...."hooray!" I also love dagobah and yoda's explanation of the force, and the interaction between luke and yoda. and lastly, the vader/luke duel, round 1.
ROTJ: I love the space battle and the throne room duel.