I think my favorite line from all of the movies is "So be it, Jedi."
My favorite part from all of the movies is definitely the part where Luke goes to see Vader, up until the very end of the movie, and I always feel like I'd love to see all of that without any of the space battle or Endor ground battle. Just Luke, Vader, and the Emperor. It's so dark and dramatic and epic. "So be it, Jedi" is my favorite line, but it wouldn't be anything special without the parts leading up to it. It needs the context of the rest of the scene.
Luke has just cut off Vader's hand, and Vader is on the ground in pain as Luke holds the lightsaber up to him, ready to make the kill.
The emperor starts laughing and then says "Good! Your hate has made you powerful. Now, fulfill your destiny! Take your father's place at my side!"
Luke looks at his fake hand, clenching it into a fist, and then looks at where he cut his father's hand off, which was also fake. He turns his lightsaber off, says "Never," and throws his lightsaber. "I'll never turn to the dark side." The Emperor just looks at him, silently. Luke continues, "You failed, your highness. I am a Jedi, like my father before me."
At this moment, the Emperor knows he has lost, and that there's nothing he can do about it, so he lets his anger take over. "So be it, Jedi."
It's by far the most powerful scene in all of the Star Wars movies, and it may be my favorite scene in any movie ever. I think there is so much in those 4 little words and how they're said. The way the Emperor says it sends chills down my spine.