I could probably just use 5 scenes from the Star Wars trilogy. So many iconic scenes.
I'd probably go further than the emperor's scene in RotJ, though, and use the entire closing sequence, where you have three different plotlines coming together simultaneously, with the Vader/Luke/Palpatine fight, the lowering of the shield on the moon of endor, and the space battle and destruction of the death star. It's just huge in scope and I just love watching that whole 15-20 minutes. "Epic" is a word that's both been bastardized and overused these days, but that 20 minutes is an epic conclusion to an epic film trilogy. It's like the final movement of a symphony.
2001 is the same, there are so many iconic moments in that film, from the apes with the monolith, to the initial sequence in space, to HAL, to the stargate.
Casablanca, too, so many iconic (and quotable!) scenes. My favorite's probably the end sequence at the airport, where Rick has Ilsa convinced he's leaving with her, only to have her leave with Victor Lazlo. Then he walks off into the fog with Captain Renault. Just tremendous. Damn, I need to watch Casablanca again now.
Andy Dufresne in the rain, after his escape from Shawshank.
The end of Se7en.
Alec Guinness, "What have I done?" and falling onto the detonator at the end of The Bridge Over the River Kwai.
Sharon Stone in interrogation during Basic Instinct! I was 18. I will never forget.