never = not ever

Let's be direct so that you will not be confused any more. You asked or said, 'For example, I think "This is the best film I have never seen" means I never saw a film better than this one before.' You are wrong. No, it does not mean that.

The others have responded, and I agree with them. I would just like to add two points: 1. It us useful to understand, or articulate, the definition of "ever": "at any time" or "at all times, always". 2. We do see this kind of construction: R.A. Butler was the best prime minister that Britain never had. Soong Meiling was the best president that China never had. It means that the writer thinks that Butler and Soong would have been ideal for the job but they were never chosen or elected to take on that job. They would have been the best that Britain or China ever had but never did.

As Jmat has said : never = not ever This is the best film I have ever seen. I have never seen a better film than this. Have you ever seen this film? (at any time in your life) No, I've never seen it. "This is the best film I have never seen" doesn't make much sense.