Hold your ground, hold your ground! Sons of Gondor, of Rohan, my brothers! I see in your eyes the same fear that would take the heart of me. A day may come when the courage of men fails, when we forsake our friends and break all bonds of fellowship, but it is not this day. An hour of woes and shattered shields, when the age of men comes crashing down! But it is not this day! This day we fight! By all that you hold dear on this good Earth, I bid you stand, Men of the West!
Here's my all time favorite: "You are the child of my makers. Inheritor of all they left behind. You are Forerunner! But this ring...is mine!"
-343 Guilty Spark
I also memorized the Prophet of Truth's speech in the beginning of Halo 3: "You are, all of you, vermin. Cowering in the dirt, thinking what, I wonder? That you might escape the coming fire? No...you're world will burn until its surface is but glass! And not even you're demon will live to creep, blackened from its hole to mar the reflection of our passage! The culmination of our journey...for your destruction...is the will of the gods! And I? I am their instrument!" That's all off the top of my head...I play way too much.
[Edited on 07.26.2010 4:05 PM PDT]