I would have to go with Aliens, A3, and A:R. Less dumb choices by characters. I rank Alien and Aliens the same. Most days I cant choose which one I like more. It goes in phases. The worst of the 6 movies listed.is A:R though. The others range from okay to amazing.
My thought on Brett is that he might have witnessed the birth, but might have still had sufficient ego to think he could probably handle himself against something he assumed was still only 2 feet long, or at least run if need be, even if he was freaked out. He may have also felt that Jonesy was the sole reason the motion tracker went off. He may have also felt embarrassed to have let Parker and Ripley down by missing the cat, and cowed to their anger.
I assume if the cat ran away, and Brett had already seen the adult he would have told Parker and Ripley to go **** themselves.