Bullitt
French Connection
Dirty Harry
Great stuff, really makes those movies. You could tack on Superfly to that series, but the movie was crap apparently.
For mainstream, anything John Williams - Even the opening sequence of Out of Africa. Star Wars is so big its easy to overlook, Indiana Jones is great etc. Ennio - 'nuff said

Also, check out the soundtracks from the Italian thriller / horror movies they were doing in the late 60s. Some fantastic stuff there, but quite avant garde in parts, and "hard" to listen to in the sense that the music does its job too well.
For something light, Topkapi is cool
Lots of movies have great songs, Breakfast at Tiffany's, Casablanca etc, but I don't count those as "musical scores" because they are stand-alone songs plonked into a movie. The Taratino stuff is in the same basket - just a lot of raiding the archives. Just like advertising executives do. Not the same as new music composed for a new movie IMO.