Maddest/most dangerous bladed weapon ever?

it is not the weapon that is dangerious, it is the person wielding it.

Flexible flail type blades or swords have their place when used by a trained combatant, but they are specialized and not as useful in real world situations today where your opponent has options.

Best edged weapons would be IMHO.

Japanese Katana-
Probably the best made, most thought out, combat sword ever created, and was outlawed in Japan because they posed a huge threat to the government.. which had rifles.

Nepalese Khukuri-
This large combat knife has been used as both weapon, cooking knife, garden tool, prybar for centuries.
The Ghurka soldiers carry these into battle today, and are known as one of the fiercest fighting groups in history.
They had no fear. (Except when the Elite Scottish Army marched through. Even the Ghurkas knew you don't mess with men who are bad enough to wear Kilts into battle. No joke.)

Tomahawk or Hand axe-
A combat tomahawk is carried today by some US forces. in close gunless combat, these are damn near unstoppable.

Bowie Knife-
Designed by Daniel Bowie bay in the day, large blade and curved hand guard was built for real world, old west action after Bowie cut his hand trying to slay a wild hog. The guard is made to catch you enemy's blade and disarm him.
Up close, these were made for fighting. Versions of the original Bowie Knife today are issued to troops around the globe.