What kind of kick are we talking about? A roundhouse, a thrusting?
When doing a roundhouse: Unless you trained it it hurts like a b**ch, and as mentioned above the foot is easily damaged too. Go with the shin, its much harder, pure bone, but train it, or you will scream like a baby.
You can use the foot, but as mentioned above it can end up in just a slap, or if you opponent leans back and only the toes hit him, it might injure them.
Or you can do a reverse roundhouse and use either your heels (needs training, the achilles-tendon is vulnerable) or your whole foot (low chance of injury, and again might only slap him). Only major drawback of a reverse roundhouse is that you turn your back towards your opponent and he might exploit that.
If you meant thrusting kicks, well the heel has tremendous stopping power if thats your goal. Low risk of injury here, but much less penetration. Its not too painful, best used to stop someone in his tracks.
Or you can use the tip of your foot (that soft part right below your toes), it has great penetration and it's quite painful also. Needs training thou or you might break a few toes.
Tell me if I missed something.