Star Wars is my favourite - first film I saw in a cinema.
In those days it was common for cinemas to scrimp on the cost of projector bulbs, so they would often project the image at half brightness. A cinema image should be bright enough to read a newspaper by, according to Stanley Kubrick, but I cannot remember seeing any bright projections until THX auditoriums appeared in the UK in the the mid-90s.
Watching Empire was not a pleasant experience on the big screen, the action between Hoth and Bespin was too dark. You really had to have the Force with you if you needed a toilet break.
I watched ROTJ more often than the other two films on VHS, for the shallow reason of enjoying the superior effects. These days I would say Empire has the best direction and acting, and is the deepest on a philosophical level, but Star Wars had an impact like nothing else.
I had the action figures of Han, Leia, Luke (and maybe the droids) before seeing the film. It didn’t spoil a thing because nobody I knew had seen it. I also read the novelisation of Empire and the Marvel paperback adaptation of ROTJ before seeing them, but again it didn’t spoil a thing.