Death's Scythe is the personal weapon of Death, the Horseman. It is one of the most powerful weapons in existence, capable of killing virtually anything — including reapers, angels, and possibly Death himself (as Dean used it to kill the original Death in Season 10, though the circumstances were unique).
Capabilities
The Scythe can kill reapers, which are otherwise extremely difficult to destroy. It can harvest souls and sever the bond between a body and its animating spirit. In terms of raw killing power, it may be second only to the cosmic-level abilities of God and the Darkness.
Limitations
The Scythe is a unique artifact — there is only one, and obtaining it requires either Death's cooperation or taking it from him. It was not readily available to hunters throughout most of the series. The weapon also carries the weight of its purpose — using it means interfering with the natural order of life and death, which has consequences.