Well i think O'Bannon and Giger need the most Credit, RS just put there ideas and design to good effect.
O'Bannon brought us a unique Alien Life Cycle.... HR Giger gave us a design to bring O'Bannons Alien to the Screen with something like we never saw before..
Getting the Monster Design was Key... HR Gigers Design was so iconic, i think it was done differently it could have had a great effect in making Alien just a typical B-Movie.
We need only look at the Original Predator Design..

Which is not to bad.... it could fit in the Alien Universe as a alternative kind of Engineer Created Monster.
But when we look at the Design they went with for the Final Predator Design and how Iconic this was, we have to ask WHAT IF they gave us that Original Predator Suit as worn by Van Damme?
And then appreciate without Gigers Art, our ALIEN Monster in ALIEN could have looked like this.
