IMDB has a list of the top 200 most iconic movies of all time.
For a discussion of what makes a film iconic, you might find this interesting reading.
For me, the answer comes easily enough. I have never seen this movie and never have an interest in doing so. I dislike gangster movies, always have… and for me the most iconic movie I’ve never seen is “The Godfather”
So, what is the most iconic movie that you have never seen?