"Make sure you're guys have the gun waiting there. I don't want my brother to come out of the bathroom with just his dick in his hand."

Hands down, funniest line in the trilogy.

Actually Sonny has a lot of them. I also like when Sonny gets mad over Mike joining the Army and then tells Connie, "Go show Carlo the tree."

I also like Clemenza: "Oh Paulie, won't be seeing him no more!" Classic.

Or Vincent in the helicopter: "I'd like to take Joes Zaza up in one of these and drop him."