My favourite Ennio Morricone is For A Few Dollars More, mainly because of the way it integrates the music box theme - especially in the final duel. Totally awesome.
Of course, Once Upon A Time In The West uses the harmonica theme to similar effect, but now every time I hear that, I just immediately think "What were the skies like when you were young?"
One of the really great thing about those films is not just the music but the silences. The opening scene and final duel in OUATITW being prime cases in point.