None of my fave three are on YouTube (that I can find).
Star Wars, small ship chased across the frame by some enormous ship that goes on, and on, and on. Think I paid five bucks at the time to see that flick, some of the best five dollars I ever spent, just for that one scene. "OK, you have my full attention."
The forty horses of the bandits hammering across the screen in Kurosawa's Seven Samurai. Breathtaking.
Opening scene to the cinematic release of Apocalypse Now. First time I'd heard speakers in the back of the cinema. Those choppers, sha-sha-sha-sha-sha...