There are two considerations when deciding which characters should use your best Bows, but they are seldom at odds. On the one hand you want your character with the highest Bow skill to carry your strongest Bow. On the other hand you also want characters without powerful magical attacks to be able to attack effectively from a distance.
If your characters are equally trained toward their highest Skill rank then there should be no hard decisions to make. Give your Bows in order of strength to your Dark Elf, Knight, Troll, Minotaur, Cleric, Vampire, and finally Necromancer.