I think the beginning of a New Hope after Vader makes his ominous entrance and says to Leia, "You Are Part of the Rebel Alliance and a Traitor! Take her Away!!" James Earl Jones voice, the breathing, that bad ass entrance. Too cool.
"You have failed me for the last time" --- Vader lines are cool, what can I say.
"I will never join you!!" Luke screams back while holding on with one arm out on the platform. His finest acting moment in the OT. Disturbed when he found out Darth is his father, his hand is gone and his life may soon be too if he doesn't decide to join the dark side.