7) Jaws (1975)
In Jaws, when Chief Brody first looks at the gigantic shark they are after, all that Roy Scheider's character can do is remain in stunned silence. "You're going to need a bigger boat," he says, delivering unscripted lines that are also reflective of how surprised he must have felt while filming this scene with the model of the shark. Well, the shark kept malfunctioning; just imagine all that frustration on set.
The deadpan response was suitable for the absurdity of the situation and captured the fear and disbelief the character would have felt. The director, Steven Spielberg, loved the spontaneity of the moment and put it in the final cut, where it has become one of the most quoted lines in cinematic history.