So many that come to mind. The scene in The good the bad and the ugly where Eli wallach talks with his brother, really quite a pivotal moment to the character. The first flight in E.T, the list goes on.
Kind of cliched to say it but, watching Blade Runner (Final Cut) again the other night and the final scene with Rutger hauer is such an amazing performance. The build up during the chase to him having compassion to ultimately spare Deckards life.
The stand off with Hau in Infernal Affairs II. The whole scene is intensified because of the relationships between the characters.