Star Wars (or Episode 4: A New Hope, as it's now known). The opening scene. You all know it. Princess Leia's corvette flies past the camera firing its guns, followed by the most awesomely oversized starship ever seen in cinema history - accompanied by the best soundtrack to a movie ever written. A scene that grabbed you by the balls and didn't let you go until the end credits rolled.
Lord of the Rings: The Two Towers. The whole of the battle of Helm's Deep. My jaw was sat on the sticky cinema floor for the whole time. I'd never seen anything quite like it before. The Seige of Gondor may have topped it in Return of the King, but we were kinda expecting it after seeing this. I never expected to actually see an army of great orcs standing - ten thousand strong - before a medieval castle, chanting like drunkern rugby fans. Oh my god, it was just the most amazing scene EVER!
And that could only be followed by the Ride of the Rohirrim as Theoden turned the tide in The Battle of Pellenor FielRAB. Yep, they just couldn't top Helm's Deep because we'd be expecting anything they could throw at us after that. But then this army of horsemen shows up and Theoden gives the most rousing war speech ever and we watch as all those horses pile down into an army of 300,000 orcs. You just can't top scenes like that!