Well, I hardly remember any lines from the previous two games but the one that comes to my mind first is Master Chief's last line in Halo 3:
Wake me... when you need me.
Some people feel he was an ass to Cortana but I personally feel somewhat sad about it. He's the reason the war ends and yet he ends up in deep space, having to sleep for an unknown amount of time. Everyone is 'happy' and safe and the hero is thought to be dead. The line was delivered perfectly.
"We don't know what they're capable of," Kurt finally told Deep Winter. "Stick with the by-the-book drills and we'll never find out, either. But put them in an impossible situation, and maybe they'll suprise us"
"Look," Deep Winter whispered. "what are those two doing now?"
Kurt smiled. "I think... the impossible"
"Chief," Kurt said, "I'm sorry that order had to come from you,"
"I understand, sir," Mendez replied. "You're their CO. You have to inspire and command their respect. I'm their drill instructor. I get to be their worst nightmare." He gave Kurt a crooked smile and climbed aboard.
I like in Halo 2 how Chief was like "You told me there wouldn't be any cameras" and Sgt. Johnson replied with "And you told me you were going to wear something nice!"
I also like the part in Halo 2 where after Chief crash lands in Delta Halo, Cortana says "Could we possibly make anymore noise
?" and then chief pulls out a rocket launcher and she says "I guess so..."
"If my face looked like a squid, I'd be angry, too!"
"I heard you fellas taste *just* like chicken!"
But here's my fav -
Cortana: I have been cooped up in here for twelve hours watching you toatie about helping that thing get set to slit our throats!
The Master Chief: Hold on now, he's a friend.
Cortana: [Sarcastically] Oh! I didn't realize! Is he your pal? Is he your chum? DO YOU HAVE ANY IDEA WHAT THAT BASTARD ALMOST MADE YOU DO?
The Master Chief: Yeeessss; activate Halo's defenses and destroy the Flood, which is why we brought the Index to the control center.
Cortana: You mean this?
[Holds up the Index]
343 Guilty Spark: A construct in the core? That is absolutely unacceptable!
Cortana: Sawed off!
343 Guilty Spark: Why... I shall purge you at once!
Cortana: You sure that's a good idea?
[the Index vanishes from her hands]
343 Guilty Spark: Why I-I...
Cortana: -Do what? I have the Index! YOU CAN JUST FLOAT AND SPUTTER!
The Master Chief: [to both] Enough!
[turns to Cortana]
The Master Chief: The Flood is spreading. If we activate Halo's defenses, we can wipe them out.
Cortana: You have no idea how this ring works, do you? Why the Forerunners built it? Halo doesn't kill Flood, IT KILLS THEIR FOOD! Humans, Covenant, whatever; we're all equally edible. The only way to stop the Flood is to starve them to death, and that's EXACTLY what Halo is designed to do; wipe the galaxy clean of all sentient life. You don't believe me? Ask HIM.
[points to the Monitor]
The Master Chief: [turns to the Monitor with a serious demeanor] Is it true?
343 Guilty Spark: ...More or less. Technically, this installation's pulse has a maximum effective radius of up to 25,000 light years. But, once others follow suit, this galaxy will be quite devoid of life. Or, at least any life with sufficient biomass to sustain the Flood. But you already knew that. I mean... how couldn't you?
Cortana: Left out that little detail, did he?
343 Guilty Spark: [to the Chief] We followed outbreak procedure to the letter. You were with me each step of the way as we managed this crisis.
Cortana: [bows her head] I'm tracking movement, we have to go!
343 Guilty Spark: [still talking to the Chief] Why would you hesitate to do what you've already done?
Cortana: We have to go! We have to move now!
343 Guilty Spark: [four Sentinels appear behind him] Last time you asked me, if it were my choice, would I do it? Having had considerable time to ponder your querey, my answer has not changed.
[the Chief grabs Cortana's chip and jams it into his helmet]
343 Guilty Spark: If you are unwilling to help, I will simply find another. But still, I must have the Index.
Cortana: [to the Chief] Get... us... out of here!
343 Guilty Spark: Surrender your construct, or I will be forced to take it from you.
The Master Chief: [through his teeth] That's not going to happen!
343 Guilty Spark: So be it.
[to the Sentinels]
343 Guilty Spark: Save his head, dispose of the rest.
[the Monitor flies away, leaving the Chief and Cortana to the tender mercies of the Sentinels]
Right after you get the Scorpion Tank on "The Ark" in Halo 3 and start driving away from the cruiser.
Ghost drives towards tank...
Marine: "Yeah, Tank beats Ghost!'
Two Hunter lumber out of the tunnel...
Marine:"Tank beats Hunter!"
Phantom Drop ship is dropping off troops...
Marine:"Tank beats everything! *Happy Laugh* Oh man, I could do this all day!"