On seeing the title, the X-Files theme instantly came to mind.
Weird.
Looking forward to suggestions. The really interesting thing about TV themes is not so much that a given theme or song or whatever is great (or even good). It's that the theme so strongly evokes the entire ethos of the series. Whether it is The Honeymooners, Dragnet, The Simpsons, Downton Abbey, Hawaii Five-O, Star Trek (the great Anthony Courage), you name it . . . as soon as you hear the first few notes, the universe of a hundred+ episodes is presented. Great feeling.