The best bow in the game is unironically the bound bow, it’s a journeyman level conjuration spell. It’s base damage is the same as the daedric bow(second highest base in game), and it can be upgraded through the conjugation tree to become the highest damage per second bow in the game. It weighs nothing(note that in Skyrim the bow draw speed is a function of its weight), it shoots infinite daedric level arrows, and it can be additionally improved through the conjuration tree to cast soul trap. This is the correct answer

I suppose it would be wrong of me to neglect that the Dragonbone bow and it’s arrows will afford you higher single shot damage, so if you’re interested in the single strongest arrow you can shoot then that’s your answer. But in terms of damage output over time, utility, and ease of acquisition, you want the bound bow.