some of my favorites

GFIII:
CONNIE:Michael…
MICHAEL:Yes?
CONNIE:Now they’ll fear you.
MICHAEL:Maybe they should fear you

right before the killing of Zasa the whole bit on the car with armand and joey's bull dog

ARMAND:Hey, Joey Zasa! Joey! Up your ass!
ARMAND:Hey Joe! Who's gonna win the car, eh? Your aunt?
ZASA:Hey. hey, why don't you get off of the fucking car, huh? You got a raffle ticket?
ARMAND:Hey Joe, take, take a good look at my raffle ticket, here we go.

GFII: when connies new boyfriend merle kisses mama corleone on the cheek the expression she makes is classic.

GFI: after tom and sonny are arguing over sonny killing tataglia and tom saying this is business not personal, and mikey starts talking about killing mccluskey and sollozzo

SONNY:You're taking this very personal. Tom, this is business and this man is taking it very very personal.

or the final time mike and sonny will see eachother at the dinner table when they are thinking of where to plant the gun

SONNY:Hey, listen, I want somebody good -- and I mean very good -- to plant that gun. I don't want
my brother coming out of that toilet with just his dick in his hands, alright?

i am sure theres more, but i dont want to bore you all