Just look at my signature for two of my favourites, also :
Burn Heretic!! - Dreadnought DOW
An open mind is like a gate with its door unbarred and unguarded - librarian DOW
The warrior who acts out of honour cannot fail. His duty is honour itself. even in death - if it is honourable - is a reward and can be no failure, for it has come through duty. Seek honour as you act, therefore, and you will know no fear - Roboute Guilliman - Primach of the Ultramarines.
Give me a hundred Space Marines. Or failing that give me a thousand other men. - Primach of the Imperial Fists
The enemies of the emporer fear many things. They fear discovery, defeat, despair and death. Yet there is one thing they fear above all others. they fear the wrath of the Space Marines - SM codex
Pain and death are illusions of a weak mind. While his gene-seed returns to the chapter a Marine cannot die. Without death pain loses its relevance. He that may fight, heal him. He that may fight no more, give him peace. he that is dead, take from him the chapter's due.
While vile mutants still draw breath, there can be no peace. While obscene heretics' hearts still beat, there can be no respite. While faithless traitors still live, there can be no forgiveness.
Consider the Predator. Let your soul be armoured with faith, driven on the tracks of obedience which overcomes all obstacles, and armed with the three great guns of Zeal, Duty and Purity. - Anon
From the darkness we strike, fast an lethal and by the time our foes can react.... darkness there and nothing more - Kayvaan Shrike
The blasphemy of the tyranids is such that only one solution is acceptable. Extermintation - Chaplain Cassius, Ultramarines Master of Sanctity