After analyzing 51 public restrooms, experts found that the stall closest to the restroom door consistently had the lowest bacteria levels and the most toilet paper. The first stall probably sees less traffic because it's near the door and people want privacy.
So, they ignore the first stall and move on to stalls near the end. In any case, when you're finished, stand before you flush. When toilets are flushed, a fine mist of water containing contagious bacteria sprays up. You can catch intestinal bugs and hepatitis from it.
Toilet paper obviously is then found in the first stall, since no one uses it. The opposite goes for urinals because it's closest to the door. Men are usually in a hurry and use the first one they can get to; the closest one. The one furthest away is the cleanest because they want to get in and get out as quickly as possible.