Beheading — full decapitation — is the kill method for vampires in Supernatural and is effective against several other creatures as well. It is direct, unambiguous, and permanent. Where many supernatural weaknesses involve specific materials or rituals, beheading is purely mechanical: remove the head from the body.
Usage
Any suitably sharp and heavy blade will work — machetes are the weapon of choice for most hunters due to their availability, weight, and cutting ability. Swords, axes, and heavy cleavers also work. The cut must be complete; partial decapitation may not be sufficient depending on the creature.
For vampires specifically, beheading is the only reliable kill method. No other weapon — not silver, not holy water, not fire — permanently destroys a vampire. The machete has become the iconic vampire-hunting weapon in Supernatural, far more so than any traditional stake.
Also Effective Against
Beheading works against most corporeal monsters as a last resort, though many have faster or more reliable kill methods. It temporarily incapacitates Leviathans (the head must be kept far from the body to prevent reattachment). Jefferson Starships (the hybrid creatures created by Eve in Season 6) could also be killed by decapitation.