Usually, the ring finger nail grows the fastest.
The best explanation I have heard is that the ring finger has TWO primary nerves running thru it. One for the left side and the other for the right side of each ring finger.
This gives the ring finger a higher electrical potential, and cell regeneration and metabolism is slightly higher where the bodies' natural electrical field is stronger.
At least... that is the only explanation I have ever read... I have no idea if its been tested.