I have several:
*"Don't worry, I have a bullet for each of you..."-Jun's Firefight voice.
*"Stick that in your ring and smoke it!"-John-117's Firefight voice.
*"They do die!"-Marines sometimes say this after you die.
*"Nothing fancy, center-mass!"-Jun's Firefight voice, and he also says it in campaign.
*Anything Buck says, because of his voice actor.
I never really got any good lines out of marines in Reach. Halo 3 had the best. There mine would be when a brute yells "We don't accept American Express!" But in Reach the firefight voice of sergeant Stacker has some great lines. My favorite is a rarer melee line "Well theres one dead bonkedybonk."
Kat FF voice- "*flirty, yet maniac giggling, then ambaressed cough* Sorry, sorry, Won't ever let that happen again.. sorry..*obviously flustered*"
Civilian. I was pointing my DMR up her left nostril(jk), in scope, and next thing I know, she goes "Is that a weapon in your pocket or are you just happy to see me?" note: I only play a female Spartan.
Edit: im a derp. i listed quotes from other games. Removed. Also remembered another Kat quote I love. Get it with IWHBYD, and annoying her (shooting, pushing, standing on her head)
"Yes, Its field tested and combat ready... and I can punch a hole through your helmet with suprisingly little difficulty...."