Trying to stay outside the box on this one and choose something I hope no one else would think of. My choice is Wabisuke!!! (aka:The Wretched One) @VinMcCarthy
Wielded by Kira Izuru from Bleach, this sword in Shikai form is awesome.
Having the ability to double the weight of anything the blade cuts, it would be quite difficult to defend against. I'd just have to make sure to keep it nice and sharp. :)