This was a ground breaking role for Sigourney Weaver. She was the lead in a science fiction action movie. Her character even created an adjective , Ripleyesque. Trinity in the The Matrix is a cinematic descendant of Ripley. A female character who takes charge and is brave and resourceful. This was new. Weaver lost out to Marlee Matlin in Children of a Lesser God. A film that has been forgotten.
I haven't seen Children of a Lesser God, but I do appreciate the significance of a deaf actress winning the Best Actress award for what I believe is the first and only time in history thus far (recently deaf actor Troy Kotsur won the Best Supporting Actor).
However, Sigourney Weaver is definitely my personal pick for 1986, and IMHO one of the greatest lead performances of all-time. Not only is it an incredibly powerful performance, which is quite rare for a genre movie, but it's full of so many brilliant details (my personal favourite is Ripley's knowing smirk when she sees that someone [i.e. Burke] has placed her gun out of reach, whilst she and Newt are locked in a chamber with a Facehugger). There's nothing surface about the performance; like every truly great performance, it feels like the actor is *living* the part.
Aliens is such a brilliant film on *so many* levels (thrilling direction/action, beautiful production design/eye-catching hardware, an amazing supporting cast, genius writing chock full of memorable quotes), but I think Sigourney's performance might be its most outstanding element, which is no mean feat.
I agree with everything you have said. Can I also add that Aliens is also be read as a commentary on the War in Vietnam. Over confident marines believe that their superior technology will overcome the aliens. They soon discover that these are not just " dumb animals". Ripley is the expert who is ignored until they realise that she understands the enemy better then anyone else.