When talking about Sergio Leone himself, Once Upon a Time in America AND Once Upon a Time in the West are shadowed by The Good, The Bad, and The Ugly mostly because it's the most mainstream of the three and it has the most famous score of the three.

Now on a scale of cinema itself, I never hear about OUATIA at ALL outside the internet. Hell, when DeNiro got his Lifetime award, the film wasn't even mentioned.