Ok I go with my 5th choice as the other 4 might make round 2. I hope.....
Take a English Statford trained actor, add the star of "Curse of the Living Corpse", some horny old graduate, and finally the BIGGEST star named Bruce. Maybe a famous director too.... Put them in a story from the Best Selling novel of the decade(or at least the 3rd bestselling of 1974) and you have the biggest blockbuster Movie since Gone with the Wind.
This is a story of character, suspense and doubt. The clash of classes and the arrogance of belief. This movie changed the holiday patterns of people for years.
Trailer
I think people were expecting a hokey B monster movie and they emerged from the theater knowing they had seen a cinematic work of art - not to mention three unexpectedly great performances by Roy Scheider, Richard Dreyfuss and -especially- Robert Shaw.
I lived 500 miles from the nearest ocean and that summer most the kids swimming in the lakes were scared.