When using a front kick which part of the foot should be the point of contact to perform the move correctly. Is it the ball of the foot or should the whole of the bottom of the foot contact?

the ball of the foot

Depends on the style, and the out come you are looking to achieve. Most styles use the ball of the foot; safe striking surface and powerful. Some styles strike with the big toe; takes a long time to develop but you obtain deeper penetration with the strike. Other styles use the full foot as more of a stopping technique, not necessarily used to inflict damage but can set your opponent up for other techniques.

It also depends on distance. Sometimes you won't have enough space to get up the speed for a damaging strike with a ball mae geri, but will with a heel mae geri.

First, it depends very much on the style. If we're talking about a traditional karate front kick, then you should be striking with the ball of the foot. I have, however, seen and heard of some karate practitioners who will pull the toes down, rather than up (almost making a fist with the toes) and striking that way.

On the other hand, if we're talking about other styles that isn't necessarily the case. In Muay Thai there is a front kick that is more of a pushing technique where the whole foot is planted on the opponent to drive them away and set them up for follow up techniques.

It really just depends on the style. Since you're posting in the karate forum though, I'm assuming you're talking about the traditional mae geri.

Since you're posting in the karate forum though, I'm assuming you're talking about the traditional mae geri.

Be careful with the usage of the word traditional. Uechi ryu is a traditional Okinawan style of Karate which uses the "toe fist" as the striking surface of the mae geri. Though I would agree the ball of the foot is the more common striking surface.

We use the ball of the foot in style of Shotokan that I practice.

maegeri has mainly 5 options, - If you consider any kick to the front a maegeri

instep, tiptoes, ball, sole, heel, depending on distance or circumstance.

There are an enormous variety of kicks out there, and any of them directed to the front of the body could be called "front kicks." As far as I'm aware, striking with the ball of the foot is the most common method of performing a front kick in karate.

Again, it all depends on the style of the individual practitioner. There are so many different styles of martial arts out there and they all do things slightly differently.

Granted, you'll get a lot of answers based on style. However, by far and away most common is the ball of the foot. It's also the safest and most useful for most people.