Blade Runner. It did very poorly at the box office, and the critics were lukewarm at best, but I loved it. I was a big fan of Philip K. Dick, so a film by Ridley Scott based on one of his novels was right up my alley. I dragged my friend to see it the week it came out, and I was blown away. Even back then I wasn’t alone. It almost immediately became a cult film that regularly played in smaller repertory theaters.
I remember reading an interview with Arthur C. Clarke back then where he mentioned that he had recently spoken with Stanley Kubrick, and Kubrick had said that Blade Runner was the most visually beautiful film he had ever seen.
I get the feeling that both the original and the sequel are the kind of film where word of mouth just doesn't do it for some reason. I had a friend years ago who mentioned that she had never seen Blade Runner, and immediately followed that by saying not to bother telling her how good it was.