The most important part is: "instrumental", I listen to music to get in the zone because the sound of a distant talk really catches my attention. Hence the same goes with music ;-) Most of the time jazz or neo-classical, but when the open office becomes noisy then it's metal (djent, black, ... )
Over time, I set up a playlist (as of now, 64h long) to not spend too long choosing something. open.spotify.com/user/sebportebois/playlist/3eKkI…
Play it in random mode, and enjoy ;-)