The lost scenes in Event Horizon. The movie as the director originally intended it must have been brilliant filming as its best. I'm a fan anyway, but it could have been much better.
THE MAGNIFICENT AMBERSONS was hacked to pieces behind Orson Welles' back and is still a masterpiece. I'm assuming his cut would've been even better!
Leslie Megahey interviews Orson Welles for The Magnificent Ambersons and his unfinished film It's All True, also, Robert Wise and Peter Bogdanovich talk about The Magnificent Ambersons (1942), from the documentary Arena - The Orson Welles Story (1982).
I would love to see the original spider pit scene from King Kong..
.... but suspect that it might prove disappointing.
Many people here seem to think that this is the original scene. It's not. This is a reconstruction of the lost scene that Peter Jackson and his film crew tried to re-create during the filming of the remake of King Kong. It's clearly a loving tribute to original, using the same techiques from the original. And what's really mind blowing is that these modern film makers found this to be one of the most challenging things they ever worked on. And this is just a one and a half minutes, with way less appealing stop motion. Just shows what a timeless spectacle the original King Kong is.