I'll give a brief list to start off, feel free to add more.
Rules:
-Can be any weapon, tool, artifact, accessory that either possesses great power or imbues great power in the weilder.
-Sentience is allowed, but being able to move/communicate like a normal person (not withstanding astral projections)
-Size should be handheld, more or less. Use your discretion.
-Nothing is to obscure (though be sure to specify the franchise if you feel it's necessary)
-Lists ranking what you consider the top items are appreciated.
Preliminary Contenders
Mijolnir (Marvel Comics)
White Lantern Ring (DC Comics)
The Black Sword (Ultima VII)
The One Ring (LotR)
The Kanohi Ignika (Mask of Life) (Bionicle Novelverse)
The Tesseract (Marvel Comics)