I first saw Westworld when I was a small boy and, boy, did Yul Brynner, with his eerie shimmery silvery eyes, scare the living shit out of me.
That film is still fantastic and takes a premis that's pretty silly if you read it on paper (and see it during the first half of the film..) but is also one of the more relentless 'chase thrillers' you'll find.
Part of the greatness of the film is Fred Karlin's soundtrack - to be fair, most of the soundtrack isn't that interesting, consisting mainly of warbling Ye Olde Wilde West-y country tracks, but when Karlin allows the electronics to come in, he hits a new level.
Here are my two favourite tracks. The first is used in the film as a sort of 'theme of the gunslinger' (as Brynner's character is called), and is just as relentless as the character is, building and building, persevering, never letting go. The second track appears during the strangely peaceful and sort of unnerving scenes when the shot or otherwise broken robots are collected by engineers to be repaired behind the scenes. It's a floaty little number, the easiest comparison might be to something like early Boards of Canada.
And here's the whole thing, if you're interested.