I may be the only person, but when I need to focus, and drown out the outside world, the best stuff for me is early Philip Glass--essentially anything up to the Koyaanisqatsi soundtrack. Einstein on the Beach, Music in 12 Parts, Northstar, etc., but as with another poste, nothing with intelligible language--so parts of Einstein don't work, but most of it is perfect.

There is something about the apparent velocity of the music that creates a bubble that the world, or even the non-focused parts of my brain can't penetrate.