#1 in 1975: Jaws

The movie Jaws had much going against it; the expensive shark robots never really worked as expected, Steven Spielberg was still somewhat untested as a director and the film went outrageously overbudget. The film's failure could have sunk Spielberg's career before it got off the ground and further dented Universal Pictures at a shaky time in its history. 
Instead it was a massive success, ushering in an era of summertime blockbuster films and propelling Steven Spielberg onto the A list. The studio would mar this success with lousy sequels made without Spielberg's involvement, but the picture's legacy is unmistakable.