The two most deadly player obtainable bows in the game are the Daedric Bow and the Nightingale Bow. However a master smith with Daedric Perks can upgrade the Daedric to provide better damage than possible with the same skill using a Nightingale Bow.
By adding a material type (Daedric) the Nightingale Bow can match the damage of the Daedric when it is improved. Bow bows have an identical base damage of 19.
In the future I will expand this project to include all of the Nightingale Weapons. Currently, this mod only effects the top tier bow. I have not tested the other four obtainable bows or any of the swords with other items of their comparable tiers. The final tier bow is the most deadly bow available in the game matched only by the Daedric.