Films with a 100% rating on Rotten Tomatoes
en.wikipedia.org
en.wikipedia.org
I have a suspicion that consumer rating of artistic content is often a function of how well the marketing matches the content to the consumer who will most appreciate it. Little or no marketing for a niche product might actually produce a higher rating from a small enthusiastic viewership than for a highly marketed product that pulls in more consumers. Of course even those that rated the content lower probably spent money on it.
I was initially sceptical, as eg the IMBD is heavily gamed towards populistic obscure cult films. But all the films in this list are indeed classics. I can really trust my profession, film critics.
These films are not the best of the best, but the classics. Which you can recommend to your grandma.