To be honest, a well upgraded long bow will serve you well into new game plus. You can find the Dragonslayer Greatbow in Anor London. It is very powerful, however heavy and uses special arrows which are rare and cost 500 souls each. There is also the composite bow, which can be powerful if upgrades, but I do not have it yet, so I can't offer many details. Good luck!

There is a better bow in the area behind the sealed door in the darkroot gardens you need 20,000 souls for the key from the blacksmith its worth it in the long run as you can get the 2k back in 2 mins check youtube for soul farming vids the guy that drops the bow is to the northeast of the stairs.

Long Bow is the best standard Bow, especially if you invest in Dex to increase Bow damage. The Black bow of Pharis has longer range than the Long Bow, but doesn't upgrade to quite as good until very very high Dex.

Pharis is in the Forest Garden beyond the Seal of Artorias (20000 soul lock). She is a phantom and appears as though she wears a ring of fog. She also has a Knight companion who spawns near her. Pharis has incredibly annoying AI which makes her roll away from you if you get too close, even off the edge of cliffs and does not respawn once killed in the current patch. Also, if hunting in the forest, do not join the Forest Covenant until after you've killed what you want.

Keep in mind currently Bows do all their damage from physical damage, so any upgrade that isn't towards +15 is a damage drop.

Barring the dragonslayer bow the best bow is the dark moon bow wihch is any bow ascended using the soul of Gwyndolin. If you combine it with the hawk ring which is in a chest behind the giant blacksmith.