these are my favorite and i would love to share it with you people:
jesus scene from the big lebowski http://www.youtube.com/watch?v=zFxqvhRyoyM i think no matter when, whenever someone says "jesus" i'll quickly say, "you said it man!" there are so many amazing quotes in this scene
revenge scene from rushmore http://www.youtube.com/watch?v=yly2UDQp6fc there are many great scenes in this film, but this one seems to stand out the most. maybe it's the who playing in the background, or maybe it's bill murray and his awesome bentley. whatever it is, it works.
final monologue in raging bull http://www.youtube.com/watch?v=KLNft6cO0I4 i don't know if you've wondered why de niro is praised so much in the film industry. if you are a little skeptical, watch this scene. better yet, watch the entire film. seriously, de niro ranks as one of the best actors ever. he practically made scorsese's movies what they are today.
that's all i can think of ... and all i can find. hope you enjoy them all!