I'm surprised nobody mentioned 2 of the most classic endings in Sci Fi, (though not technically the last scene of each movie, it is the defining & climactic moment the entire movie was building up to:
Close Encounters of the 3rd Kind - the Mother Ship appearing & turning out to be bigger than Devil's Tower, plus the scene where the aliens select Roy Neary to go with them over the handpicked agents, as well as the last alien communicating via sign language...and a smile.
Superman II - not the scene where the bully who beat up Clark gets his, (though that was good too), but the scene where General Zod makes Superman take his hand & "declare eternal loyalty to Zod". Then when Superman takes his hand, crushes it, lifts Zod over his head, gives HIM that smug cocky look he had for most of the movie, and then flinging him across the Fortress, wow that was awesome. Everyone was waiting for Zod to get his the entire movie & when he finally got it, it was worth the wait!