You can't become a demon that way, you have to have at least 95 corruption and ask Lyssieth to turn you into one.
The best weapon in the game depends on what cost are you comfortable with.
The overall best in my opinion is the Knuckle Duster, it has zero cost to use and is easily obtainable.
The clear demonstone is slightly stronger, comes with a spell but it's a lot more expensive, otherwise it's pretty much the same.
The real power comes from having 100 physique (only unarmed gets scaling from physique), getting all unarmed damage perks, getting all corresponding elemental perk and getting the perk from Hannah's training.
I've posted screenshots of the damage before and the Knuckle duster is stronger than the Zweihander when your Physique is 78 or higher.
Weapon comparison
I used cold elemental because few enemies have shielding against cold, the cold elemental has two forms (Ice and Water, each with their own resistances), the perk for learning 3 cold elemental spells is great and also because it's cool, literally.
There are weapons that can cause even more damage than this but they do come with some costs.
Maybe I'm too much of a cheapskate to use them