Knifes
Knifes: An instrument for cutting, consisting essentially of a thin, sharp-edged, metal blade fitted with a handle.
When choosing such a weapon maim/kill a person or animal, one should go with a quality knife that most armed forces use. That's right, the good old KA-BAR. Used primarily by the US Army and USMC, it is the highest standard in strength, weight, sharpness and reliability. It is named the KA-BAR after being attached to the BAR heavy machine gun for use as a bayonet. They range from prices from around $20-$90, however, their raw quality makes it worth it. See http://www.grunt.com/ for some, or Google "KA-BAR".
As far as selecting a proper military knife to go with, you may want to look in the antique section. Modern bayonets and knives are serrated on the back, and while it will do absolutely terrible damage to your opponent going in and coming out, it also exponentially increases the amount of shit you'll be in with the law. Try finding turn of the century rifle bayonets, from The Great War, World War II, Korea, and Vietnam. Generally the blade gets shorter and cleaner the more recently it was made. If you've ever seen rifle bayonets from the Great War, they're really quite horrifying. 18 inches long, blood grooves, sawbacks, etc. If you're into mutilating your opponent as well as killing him, look here.
When you actually have one, you should aim for such weak points of the human anatomy as: neck, face, joints, kidneys, stomach, subclavian artery, or Achilles heel. If you go for the neck, the victim dies fairly quickly. If you go for the face, you probably blind or incapacitate him. If you go for the joints, that's limb he can't use. The kidneys, because the intense pain will keep him paralyzed and silent until he quickly bleeds to death. Lol. The stomach, when attacked generates a huge amount of shock and the victim will typically crumple to the ground, or stop fighting and touch his wound in disbelief; expect a man to die from a stomach wound in 15-45 minutes (this is a particulary painful death since the stomach acids are destroying his flesh as he dies). The subclavian artery is located between the collar bone and the shoulder blade. Stab downward and pull the knife out with a slashing motion and this will sever the artery as it is about two inches beneath the skin. Expect a man to die from a subclavian artery slash in about 30 seconds. No exceptions (There is absolutely no way to stop the bleeding from this wound even if he did get to a hospital in time). Then finally the Achilles heel, so he has to crawl away from your threatening demeanor.
We're not liable if you do any of this, of course. Ergo, you really shouldn't do it.