Anything made by Masamune or Kagehide.
The Honjo Masamune and Saddle Cutter in particular make me think badass cranked to 11. Saddle Cutter has another less PC name and a grim background to go with it, but I consider it the pinnacle of what sharp means in the context of Japanese design and craftsmanship.
Date Masamune struck a fellow mounted (and armored) samurai with such a downward hit he cleaved the guy in half, embedding the tachi in his opponents saddle frame.
That's fucking sharp.