Luke: No. You're coming with me. I'll not leave you here
Princess Leia: But, why must you confront him?
Luke: Your thoughts betray you, Father. I feel the good in you, the conflict.
Vader: There is no conflict.
Luke: You couldn't bring yourself to kill me before and I don't believe you'll destroy me now.
and I thought this one came out poorly until recently. It's the climax of the trilogy and my #2 top Star Wars moment (if you want the full list "ask me again some time"):
Luke: You've failed, Your Highness. I am a Jedi. Like my father before me.