The creed of the Grey Wardens: "In war, victory. In peace, vigilance. In death, sacrifice."
The Warden Oath, spoken at the Joining:
Join us brothers and sisters. Join us in the shadows where we stand vigilant. Join us as we carry the duty that can not be forsworn. And should you perish, know that your sacrifice will not be forgotten. And that one day we shall join you.
The Dalish Elves have the "Oath of the Dales";
We are the Dalish. Keepers of the lost lore, walkers of the lonely path. We are the last of the Elvhenan, And never again shall we submit.
From the Chant of Light, notably spoken by Knight-Commander Meredith in Dragon Age II:
Blessed are the peacekeepers, the champions of the just. Blessed are those who stand before the corrupt and the wicked and do not falter.
In the Dragon Age II DLC quest Legacy, Malcolm Hawke, the protagonist's father, also made one into a Badass Prayer:
"Be bound here for eternity, hunger stilled, rage smothered, desire dampened, pride crushed. In the name of the Maker, so let it be."
Malcolm Hawke's own personal creed:
"Magic will serve that which is best in me. Not that which is most base".
In the DLC Mark of the Assassin, Hawke proves that badass creeds run in the family, stating their primary motivation for their actions;
Amonkira. Lord of Hunters. Grant that my hands be steady, my aim be true, and my feet swift. And should the worst come to pass, grant me forgiveness.
Paragon Shepard gives their character defining line in 2, while invoking it in the Refusal Ending in 3.
Shepard: I won't let fear compromise who I am.
The eponymous Assassins Creed: "Nothing is true. Everything is permitted." It's the main quote on the Badass Creed page and was in fact the actual creed of the historical Hashashin sect, led by Hassan al-Sabbah, from the name of which we get the word "assassin".
Note that the AC1 version has three tenets in addition to this motto: stay your blade from the flesh of an innocent, discretion in all things, and never compromise the Brotherhood. This is a major plot point, as Altaïr's violation of all three at Solomon's Temple led to his demotion, hence his starting as a Novice and having to literally kill his way back to Master Assassin.
It is expanded in the sequel, in the scene where Ezio is formally sworn into the ranks of the Assassins. It sounds even cooler that way, once you know exactly what it means... although the "three tenets" are now nowhere to be seen, possibly due to Altaïr's reforms. (Amusingly, it's also how to solve the last of Subject 16's Glyph puzzles.)
"Laa shaya waq'in moutlaq bale kouloun mounkine." (Arabic, literally "Nothing is absolute; Everything is permitted.")
Machiavelli: "Where other men blindly follow the truth, remember:" Ezio: "Nothing is true." Machiavelli: "Where other men are limited by morality or law, remember:" Ezio: "Everything is permitted." Machiavelli: "We work in the dark to serve the light. We are assassins." All: "Nothing is true. Everything is permitted."
The central tenet of the Creed is even more impressive when you consider that it's essentially the answer to an existential crisis, an individual's realization that there exists no rational force governing nature: If there is no universal truth, nothing limits what you can do. And all of this centuries before philosophers would assemble the actual school of thought.
Altair: I understand now that our Creed does not command us to be free. It commands us to be wise.
Ezio: To say that nothing is true, is to recognize that the foundations of society are fragile, and that we must be the shepherds of our own civilization. To say that everything is permitted, is to understand that we are the architects of our actions, and that we must live with their consequences, whether glorious or tragic.
While most of the cast in Team Fortress 2 carry a Badass Boast in their "Meet the..." videos, the Sniper has a code. "Professionals have standards. Be polite, be efficient, have a plan to kill everyone you meet."
"Have a plan to kill everyone you meet," became the official slogan for US Marine India Company's Forward Operations Center, out of Fallujah, Iraq.
"I am he who watches they. I am the fist of retribution. That which does quell the recalcitrant. Dare you defy the Warchief? Dare you face my merciless judgement?"
The Horde in general has: "Lok'Tar Ogar! Blood and Thunder! My life for the Horde!", although the entire creed is rarely ever spoken in its entirety, except during a quest to grant the Taunka membership in World of Warcraft..
High Commander Halford Wyrmbane: Through the valleys and peaks of Mount Hyjal, across the shifting sands of Silithus, against the Legion's dread armies - we have fought. We are the nameless, faceless, sons and daughters of the Alliance. By the Light and by the might of the Alliance, the first strike belongs to us and the last strike is all that our enemies see. We are 7th Legion.
Reversal from Hitman 2, and possibly Real Life: "I don't know anything, I didn't see anything, I wasn't there, and if I was there I was asleep." A reversal because, rather than pointing out that people should be afraid of the speaker, it's pointing out that he should be afraid of other people.
Or, as we used to know it, "My name is Atma... I am pure energy... and as ancient as the cosmos. Feeble creatures, GO!"
The title character of Mega Man Zero gives us this little gem, said at the end of the fourth and final game (of the series):
Zero: "I never cared about justice, and I don't ever recall calling myself a hero... I have always only fought for the people that I believe in. I won't hesitate... If an enemy appears before me, I will destroy it!"
In Modern Warfare 2, Captain Price gives an epicly poetic-awesome speech in response to General Shepherd betraying him & Soap:
"The healthy human mind doesn't wake up in the morning thinking this is its last day on Earth. But I think that's a luxury. Not a curse. To know you're close to the end is a kind of freedom. Good time to take...inventory. Outgunned. Outnumbered. Out of our minds. On a suicide mission. But the sand and the rocks here, stained with thousands of years of warfare...They will remember us. For this. Because out of all our vast array of nightmares, this is the one we choose for ourselves. We go forward like a breath exhaled from the Earth. With vigor in our hearts and one goal in sight: We. Will. Kill him."
Dawn of War is full of these, which is entirely fitting given the source material. Particularly common for the Space Marines, obviously - "While the enemies of the Emperor still draw breath, there can be no peace", for example.