Depends on how my mind is. If I need to concentrate because I am anxious, I try to put on something without serious lyrics, though gentle enough to calm my anxiety down. If I am calm yet finding it hard to focus, I tend to put on light music that I usually listen to, like my most listened to tracks: I find these tracks are the easiest to drown out with the least distractions, as I already know in my head where the tracks are going, however with new tracks even instrumentals when I lose concentration I tend to focus too much on the new tunes and such.

Many times I don’t listen to anything as my head is too cluttered that way.