External Noise distracts me the most because it affects either what I am hearing or the temperature. A way I ensure focus when there is a sound of some sort is just by trying to ignore it or even cover my ears. The way I ensure focus when my surroundings are really hot or really cold is by either putting on a sweatshirt when it's cold or putting on a short sleeves shirt when it's hot. If clothes don't help, then I try to fight through them.
I completely agree that external noise is the most distracting; good to include temperature as a form.