There have been millions of uses of special effects in the movies. Among the standouts are the cow being hit by the car in O Brother Where Art Thou, Tom Hanks shaking hands with three presidents in Forrest Gump, and Sean Connery's wig staying on underwater in Never Say Never Again.
But for me, I don't think anything will ever top the skeleton army in Jason and the Argonauts. Ray Harryhausen's piece de resistance of seven stop-motion skeletons battling live actors took four months to complete, and it was time well spent. It wouldn't take anywhere near that long with computers today, but I can't see how all the technical improvements made since 1963 could match the sheer energy of this encounter.