I think the worst line in Return of the King is the cheesy "I am no man" line.
The delivery could been better, but again, compared to the source material:
"Hinder me? Thou fool. No living man may hinder me!"
Then Merry heard in all sounds of the hour the strangest. It seemed that Dernhelm laughed, and the clear voice was like the ring of steel.
"But no living man am I! You are looking upon a woman. Eowyn am I, Eomund's daughter. You stand between me and my lord and kin. Begone, if you be not deathless! For living or dark undead, I will smite you, if you touch him."