Personally mine is the speech R’tas Vadum gives in halo 2 before you first play as the arbiter. “On the blood of our fathers, on the blood of our sons” a close second would have to be Sergeant Johnson when he drops Chief the tank. “I know what the ladies like” as he loads the machine gun.
Only thing that comes to mind is a line from Call of duty three that goes something like “your no good to me dead.” Not sure how it goes exactly but it is along those lines.
EDIT: Seeing where the OP changed the thread title I will go with “Child of my enemy why have you come? I offer you no forgiveness.” There is more to it but that is the part I like.
Here are my favorite video game quotes:
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“My friend’s trust is not the issue today, Spartan Locke. It is my trust you must earn.” from Halo 2 Anniversary
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“Good, the bridge is down. Now about those Wraiths.” from Halo 2
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“It was my job to take care of you.” from Halo 4
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“We were supposed to take care of each other. And we did.” from Halo 4
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“Stop firing rockets at my house!” from Grand Theft Auto 5
Oh sorry guys I didnt clarify I dont make many posts I am so sorry I meant any Halo game lol
Edit: now the title is fixed again sorry for miscommunication 
Still my favorite quote from any Halo game is from H3: “were it so easy”.
I’ve explained this before, but I’ll give the short version here: it’s said by the same character with two different meanings. The first almost feels like a challenge. The Arbiter basically telling the Chief “come at me bro”. The second is him holding out hope that his brother in arms is still alive. The poetry and symmetry of this quote impresses me.
“Men, we led those dumb bugs out to the middle of nowhere to keep ‘em from gettin’ their filthy claws on Earth. But, we stumbled onto somethin’ they’re so hot for, that they’re scramblin’ over each other to get it. Well, I don’t care if it’s God’s own anti-son-of-a–Yoink- machine, or a giant hula hoop, we’re not gonna let 'em have it! What we will let 'em have is a belly full of lead, and a pool of their own blood to drown in! Am I right, Marines?” -Johnson
Where it so easy. Impactfull and easy to use in daily life.
“I’m ready! How about you?!”
Dude I get chills every time I hear this. Broham wasn’t afraid of death. He jumped into it headfirst.
“Send me out… with a bang.” is a close second.
Gotta go with the classic “Were it so easy”. Goes to show Thel wouldn’t have gone down without at least a good fight.
“THis cAvE is NoT A naTuRAL forMaTIoN”
-Cortana 2552
I don’t know if I have a favorite per say but what came to mind first was,“Don’t shake the light bulb.” 
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Then when Cortana is like this cave isn’t a natural formation so it must lead somewhere… I am always like really Cortana?? This incredibly smooth rock/concrete tunnel with lights isn’t natural? Thanks captain obvious… 
basically all the halo 2 quotes deserve a mention but i would choose when Chief says “To give the Covenant back their bomb.”
“Good thing the food nipple’s waiting for me back at the star ship, 'cause man, have I worked up a big, Grunty, thirst!” - Thirsty Grunt in Halo: CE
What if you miss?
I won’t.