I don't know that music itself can be scary, though there are plenty of dramatic moments that make you jump (if you don't know the piece, that is). For example, Stravinsky's Rite of Spring has several such moments. Andy above mentioned Mars from The Planets but I always used to think, when I was younger, that Saturn was scary; Mars was just brutal.
And would kickemon and korgy think that the music for Tales of the Unexpected was scary if it had different associations? Supposing it had been used as a sig for, say, a holiday programme?
Having said all that, in the spirit of the question, my vote would go to a piece called Escape Velocity by Trevor Duncan, which was used as the opening and closing music for Quatermass and the Pit (the TV series, not the film).