Theoden has the best dialogue in Lord of the rings
Theoden hems and haws, but few can turn a phrase like Theoden King.
When first we meet Theoden, he is a shadow of his former self. He has Grima Wormtongue as his advisor/manipulator. Saruman has an iron grip on his mind. It is unknown how long Theoden as been under the spell of Saruman.
It takes Gandalf the white to drive Saruman out and get Theoden back to his old self.
Your Fingers Would Remember Their Old Strength Better, if They Grasped a Sword-hilt - Gandalf to Theoden
At the funeral of his son Theodred.
Simbelmyne. Ever has it grown on the tombs of my forebears. Now it shall cover the grave of my son. Alas, that these evil days should be mine. The young perish and the old linger. That I should live to see that last days of my house.
And when Gandalf responds with Theodred's death was not of your making. Theoden says something that has always stayed with me and is delivered with such anguish and grief.
No parent should have to bury their child.
About halfway through the battle of helm’s deep.
Théoden: So much death. What can men do against such reckless hate?
Aragorn: Ride out with me. Ride out and meet it.
Forth Eorlingas
The horn of Helm Hammerhand shall sound in the deep one last time. Let this be the hour when we draw swords together. Fell deeds awake. Now for wrath, now for ruin, and the red dawn. Forth Èorlingas!
When he responds to Aragon berating him telling him that Saruman comes not for the crops of Rohan, but for it’s people, Theoden responds with
What would you have me do? Look at my men. Their courage hangs by a thread. If this is to be our end, then I would have them make such an end as to be worthy of remembrance.
When Gondor lights its beacons asking for aid and Aragorn sees it from the outskirts of Rohan, and runs up the stairs letting Theoden know, Theoden delivers the perfect response after a pregnant pause.
Aragorn: Gondor calls for aid.
Théoden: And Rohan will answer.
and finally exhorting his troops before charge at the battle of Pelenor fields when they reach the outskirts of Gondor
Arise, arise, riders of Théoden! Spears shall be shaken, shields shall be splintered! A sword day, a red day, ere the sun rises! Ride now! Ride now! Ride! Ride for ruin and the world’s ending! Death! Death! Death! Forth Èorlingas