Palm Heel or Punch
The palm heel is powerful and offers the least chance of injury. The punch is more dangerous since any improper part of the technique may result in injury (such as a bent wrist my sprain or a loose fist may break a finger). Both of techniques use a thrusting movement, the only difference is the contact surface and how you hold your hand. With the punch, contact is made with the first two knuckles, with the knuckles in line with the bones of the wrist and arm. Use hip snap and keep the elbow close to the side to preserve linear movement. To help with this, just keep your arm so that it brushes against your side as you strike.
The basic "karate chop." It is ideal for penetrating strikes and looks more "professional" and impressive than a hammer fist. Knife hands must strike through the object so aim past the object. Start with the hand fully counter rotated so it can snap into the technique. Strike with great speed and tense the knife hand at impact.
A front or downward elbow strikes are the easiest arm breaks to use. There is less chance of injury than if using the hand.