The Kookaburra Outdoor Cinema near Mundaring Weir would have to be my favourite. It's location, surrounded by bushland in the Beelu National Park, is magical, and it's just walking distance from the lovely bush camping ground at the Perth Hills Centre. All in all, going there to watch a movie in the evening is a perfect thing to do after an energetic day hiking in the bush.
Movies by Burswood, located next to Crown casino, is my favourite outdoor cinema. They cater towards families and all money raised goes towards childrens charities. They rely on volunteers to assist in the nightly screenings. I found the ticket prices were slightly lower than other cinemas as well.