Thai roundhouse to either the leg ,the ribs, the neck or the head...Mines fast strong and I roll my shin on impact just to do that little bit more damage.....muay thai bread and butter.
Some interesting points here. However, if you are talking about which is technically the most powerful kick it is either the front thrust kick or rear/back kick. This is due to the alignment of your hips ensuring that, if executed correctly, the whole of your body weight will strike with the kick.
While the Roundhouse or spinning hook kicks etc are potentially more destructive they rely on speed of the spin, speed of the leg/foot and weight of the leg. However, because they are traditionally used against a target higher up the body which are more vulnerable (ie the head) they can be more devastating in their application.
spinning back kicks rely on the thrust also just like the front, side, and back thrust kicks. The Spinning hook kick does rely on a lot of speed and snap on the hook.