To me, it has to be Pretty Woman. Not Julia Roberts' first film, but the film that immediately catapulted her to a box office start, if not the star of that era. She lit up the screen with that smile, charmed the hell out of the audience by blowing her nose loudly into Barney's handkerchief (kudos to Hector Elizondo in that scene, and every other scene he was in), and dropped jaws in that red dress. This movie was the arrival of Julia Roberts.
What other movies can this be said of?