The strength of a kick is dependent not just on the type of kick, but also the strength of the person performing it, as well as how well they perform it. However, some of the strongest kicks in Taekwondo are the back kick, the side kick, and the reverse turning kick.
A reverse turning kick can be a very powerful technique. It’s relatively easy to increase the speed at which a reverse turning kick is performed, and thus increase the momentum of the kick. The back heel is a tough, bony part of the body, and when hitting an opponent at high speed, will cause a lot of damage.
The disadvantage of a reverse turning kick, however, is that it is generally harder to balance while performing it – particularly at the moment that the back heel strikes the target.
Side kicks and back kicks are relatively similar in their motion. However, it’s generally harder to perform a side kick with the right technique than a back kick. For someone who has not had much training in Taekwondo, it’s generally easier to perform a powerful back kick than it is a powerful side kick. It’s also generally easier to perform a kick with the heel than the foot-sword (if the back kick is performed with the heel rather than the foot-sword).
Thus, for a well-trained Taekwondo practitioner, these three kicks will be the most powerful ones they can perform. However, a well-trained practitioner can perform a front snapping kick more powerfully than a complete beginner can perform a back kick, so training is more important than which technique you choose.