OK so I tried to separate all weapons in three groups, good normal and bad. I think this is quite difficult for most of them since they are almost all balanced.
A tier (good weapons)
-Sai (fast, very accurate, disarm, multihit, block)
-Knife (fast, very accurate, multihit, good damage with MOA)
-Long Sword (multihit, great damage and good range, only weapon in the game to get benefited from Master of Arms and Strong Arm at the same time?)
-Baton (multihit, high countering chance, block, disarm)
-Whip (multihit, high countering chance, disarm)
-Scourge / Flail (highest multihit damage, kinda bad accuracy)
-Sabre (fast, multihit, good damage)
-Broadsword (fast, multihit, good damage)
-Lance (counter, disarm)
-Leek (average damage, enormous amount of hits (?) )
B tier (average/normal weapons)
-Hammer (bad accuracy, low range, highest single hit damage)
-Fan (very fast, very low damage, lots of hits that make it useful when a brute has sabotage)
-Hachet (similar like the broadsword but kinda heavier and slower)
-Shoriyuken / Ninja Stars (low damage but good anti pet weapon, can kill 3 dogs and a wolf in one attack)
-Trident (good damage, counter, kinda slow and not very accurate)
-Halberd (great damage, counter, slow and not much accurate)
-Frying Pan (good damage, average accuracy)
-Trombone (good damage, kinda slow)
-Coffee Cup (good damage, bad range (?))
-Mammoth's Bone (average damage and accuracy)
-Iron Mace (good damage, average accuracy, kinda slow)
C tier (bad weapons)
-Bumps (worst accuracy)
-Keyboard (really low damage)
-Tennis Racket (really low damage)
-Noodle Bowl (kinda bad damage, very slow (?) )
-Piou-Piouz (bad damage (?) )
The (?) sing means that im not sure about the real power of those weapons since they are very rare.
Basically the best weapons are the fast ones and with good accuracy. Their damage doesnt mean that much at high levels. But accuracy is crucial because weapons like the bumps can easily be avoided by high agility brutes, unlike fast weapons like the Knife or the Sai. Multihit is also very important. Weapons like the Knife or the Leek can do and incredible amount of hits in one attack. Countering weapons like the baton are really good and useful too.
The Sai for me is the best weapon in the game because its fast, multihit, has good accuracy, does decent damage, and can both disarm and block.