That's really something you could have looked up... but, hey, whatever.
The strongest weapon in Fable 3 is Jack's Hammer, if that's what you were really asking. Otherwise: The strongest sword is a toss up between The Swinging Sword and Avo's Lamentation. Avo's Lamentation does less (but comparable) damage to The Swinging Sword. It's worth noting that Avo's Lamentation actually has a higher base attack power then is shown because it does extra damage to EVIL enemies. Which basically includes every enemy in the game. In addition, it also sometimes electrifies enemies. Enemies that are electrified cannot attack or block and you're free to simply beat on them as much as you want. They take extra damage from being electrified as well.
The strongest gun is The Black Dragon, but its attack power only exceeds every other gun during night time. Otherwise it's a toss up between the Dragonstomper .48 and the Chicken Bane.