At one point, just about every kid my age owned the Factors Hildebrant Star Wars poster. That, and those Farah Fawcett posters were everywhere.
I still have a copy of the Star Wars in terrible condition, but I can't part with it. Most are lost to time.
So if the question is the most owned of all-time, especially in 1977, 1978 and 1979, the Factors Star Wars poster is my answer.
If the answer is the most owned in collections of people who frequent this website, then I agree with the previous answers. It would be Star Wars, Jaws or BTTF.