So a lot of the guesses here are speculating around things like fire or electricity, in truth these aren't terribly painful ways to die. After a certain temperature/current threshold the nerve damage is so complete and instantaneous that there isn't much capability to even feel pain. I'm going to assume that most of the magic and psychic hotboxes in smash work in a similar fashion. Most of the super blunt force trauma moves like D3s hammer, or Bowser's f-smash, are so violent that they are likely to cause instant death. If the death wasn't instant then, then shock and blacking out are inevitable before death.
Long bladed weapons like Marth/Link/Ike sword are likely to sever an artery on a clean or limb hit. Since they are all mainly slashing weapons, we'll assume the lethal blow isn't mainly a piercing motion. While these types of slashing wounds wouldn't be a pleasant death, you'd mercifully bleed out and lose consciousness in less than a minute.
Taking all this information into account, I have to assume that the worst way to die in smash brothers is to fall victim to this torture device.
The blade design is near useless for stabbing, and the pronged blade design make it clear that it was designed to land a clean killing blow. Intead Metaknight's sword is designed to kill by inches, rending and maiming muscle while inflicting superficial glancing blows. Even if Metaknight lets you leave the battlefield breathing, the ragged lacerations and puncture wounds inflicted would be a nightmare to try and close, almost guaranteeing either a slow bleed out or protracted death by blood infection after the injury rots.
The Geneva convention outlaws conical style bayonets because they are designed to kill off the battlefield in the ways mentioned above. I think Metaknight's probably a war criminal for bringing that sword to smash.
