Peter Falk as Columbo. Hard to imagine the initial pick for the role, Bing Crosby, doing the Columbo shtick. Same with Carroll O'Conner as Archie Bunker. Mickey Rooney was originally planned to play Archie.
Robert Englund will always be Freddy Kreuger and Harrison Ford will always be Indiana Jones. I doubt anyone will ever try to reboot Rocky. They will never capture the mood nor the intimacy Stallone put into his creation.
I'd bet every high school in America had a kid who could have played Mr Dynamite by just being himself. I know my school did, and that was 30 years before the film. Our guy showed up at our 40th class reunion and he hadn't changed.