Alien is a great movie.
Predator is a great movie.
Alien3 (Assembly Cut) is an underrated movie and should be seen by those who hate the theatrical edition.
Predator 2 is a good follow up but not on the same level as the original.
The rest are a mixed bag of average to poor movies.
I have lost count how many times I have watched both Aliens and Predator 1 but every time I sit down to watch either of them I am gripped to the ending credits. I can't choose.