Poledouris’ Conan the Barbarian turned what could have been a really cheesy film into a fantasy epic. Can you imagine the film without Riders of Doom?
I think the movie would have been A-OK even if Poledouris hadn't scored it. Milius was firing on all cylinders here. That being said, I sure am glad Poledouris was able to work his magic on it. He certainly nailed it.
Goldsmith nailed Basic Instinct. Every note is perfect for the movie. I can't imagine any other music for the film. Same for L.A. Confidential.
I was going to narrow the field to "When someone redoes the movie and the music HAS to come with it." But then I realized that that was going to include only films that were going to be remade / rehashed.
Even in that narrow band you can still split it into "People are going to notice the James Bond / Superman / Star Wars / Psycho / Mission Impossible / Star Trek theme isn't there" as opposed to "Hey, they included 'Windmills of Your Mind' in the dance scene, isn't that cute?"
In the case of the recent Mulan score Goldsmith didn't make the cut. (But in Ralph Breaks the Internet he did.) I didn't miss his score so much as I missed him, if that makes any sense. I didn't need his score to be referenced but I needed something to be as good as his.
The Mark of Zorro is amazing (and Newman nailed it) but I can't imagine anyone referencing it.
I'm trying to get non-Williams examples here, but the new Lost in Space show including Williams' theme is incredibly satisfying. (And Christopher Lennertz nailed it.)