"Hard Boiled" by John Woo definitely has my vote as one of the best action movies ever made.  I was lucky enough to see this in a theater when it came to America.  Lots of scenes to choose from, but this is one of the best.  The two protagonists enter a hospital to attack the bad guy of the film and just do a monumental amount of damage.  Check out how the majority of this scene is just one long shot.