Occasionally across RPG titles there are pieces or sets of armor that, when equipped (or equipped in a complete set) grant the wearer nigh-invincible levels of defense to all forms of damage and complete status immunity, often with additional effects, like continuous HP/MP renewal. I can't think of a specific one to point at right now, but I'd probably go for something like that.
Or, if pressed for a specific item, I'd probably go for one of the SnapShot/SnapDragon swords from the Tactics Ogre Games. They're made by sacrificing a person on your team and transforming them into a sword, which generally has the highest attack rating of any weapon in the game and gets half of all the stats of the person that was used to make it. And any special abilities they might've had as a plus. So I'd go with something like that, with the weapon having been snapped out of someone that was exceptionally strong in many areas and with a few cool powers.
And have it soulbound, and unusable by any other living being.
Kind of like Sasori of the Red Sand and his Third Kazekage. It's really strong in a lot of ways, retains its funky powers from when he was alive and, because of the nature of the weapon (creation/use/etc), he's probably the only one capable of wielding it at all.