Frequently Asked Questions:
A: Check item meta-stats under Community for a list of weapons; the rarest ones are best.
You may mean, "which is the best weapon I can buy RIGHT NOW" in which case the answer is, whichever gives you the best damage/cost ratio. If you have only a little money, pick the best from the store. If you have more money, pick the weapon with the highest base damage and upgrade it at the blacksmith. If you have a LOT of money, find someone to sell you one of those rares... :)
A: Multiple strikes are always directed at the same opponent. This is for balance. It also prevents (potentially) even more confusing behavior when Vampiric Aura is involved. :)
A: One list can be found in the Item meta-stats
link on the Community Info page.
More info can be found in the Wiki