I'd put lots of music created by Ennio Morricone for many films right up there.
'Once Upon a Time in the West' I'd put right up there....the entire score for that film.
Other notable films would be 'The Good, the Bad, and the Ugly', 'A Fistful of Dynamite', 'My Name is Nobody', and 'Once Upon a Time in America'. All Sergio Leone films as it happens, although Morricone has also done great music for many other good films too, one example of which is the music for a film called 'Burn'(Queimada) made in 1969 which I'd imagine not many people will have heard of. Very good use of music for that film that suited it well..
Other stuff I'd put right up there would be ......
John Barry's score for 'On Her Majesty's Secret Service'. 'We Have All the Time in the World' was used for great effect right at the end of the film.
Maurice Jarre's main theme for 'Lawrence of Arabia'.
Paul Giovanni for 'The Wicker Man'. The entire score.
Roy Budd for 'Get Carter'.
The Soundtrack for 'Nosferatu' (1979 version).
Nino Rota for 'The Godfather'.
Bernard Herrmann for 'Taxi Driver'.
Krzysztof Komeda for 'Rosemary's Baby' and 'The Fearless Vampire Killers'.
I've mainly listed entire scores, as the ones I've listed have used music for the best effect throughout the entire film and there have been several pieces of music from that score which worked really well within the respective film.
One example of just one particular individual piece of music used within a film for best effect would be John Murphy's 'In the House - In a Heartbeat', used in '28 Days Later'.
And one more,..... 'O Fortuna' used in 'Excalibur'.