The score adds to the wonderful script and Leone's sublime direction, and they come together perfectly. Even if it had a lesser score, it would still be my favourite western.
To be slightly anal I thought I'd bring up the point that with this film in particular, the script adds to the score.
The reason why I say this is because Sergio Leone stated that with this film he built the film around the score composed by Ennio Morricone which came first.
The two are tightly intertwined. To answer the other poster above who said that this film would be nothing without the score. Technically that may be right as the film is built around the score and possibly wouldn't even exist without it. Or at least it would possibly be an entirely different film altogether. It wasn't a case of adding some general music to the film. It was done more the other way around. Made as much like an opera as it was a film.