I just watched the movie. It is the last line of Dirty Harry.
Dirty Harry is a great movie, and that is a great scene, but there are a couple of things wrong IMO with including it in this list.
For one, the list is about the best closing line, not closing scene in a movie. I don't think "Well, do ya punk?" by itself really is all that great.
Secondly, the movie doesn't exactly end with that line; there's still another 1-2 minutes left where Dirty Harry takes out his ID and badge, contemplates on it for a second or two, then throws the badge in the water and walks away from the scene and away from the audience. That lends the gravitas to the ending scene moreso than the line(s) spoken 2 minutes earlier.
Here's the rest of the scene after the last line is spoken and the perp shot:
rosebud is the closing line as the sled with the name on it is the closing line. the words were just said in the beginning of the movie. and you hear them as you see the sled.
I like the last of Five Easy Pieces where the trucker asks Jack Nicholson where his coat is, says it's gonna get cold where they're going. Love that movie.