Re: What is the best kicking martial art?
For the most part, "what kicking art is the best" becomes a redundant term when you don't take into account the expertise, skill and condition of the person using such art. There have been many issue of style vs style match-up and you will always get mixed results of one practitioner beating another in a style vs style match up.
The main question would be "what are you trying to achieve":
Muay Thai and Kyokushin combined basic skill with very hard hitting techniques - you don't need to be very flexible to be able to perform a good teep, roundhouse etc but can still deliver hard power with practice. You also won't see a Muay Thai or Kyokushin fighter really taking a backward step, both are a solid stance martial art.
Savate and kickboxing again have the same basic techniques as the above but are more fluid in terms of the stance and movement.
Some Karate styles and TKD are more of a "bouncey" (I use this term loosely) martial stance geared around points fighting but can still deliver hard kicks.
At the end of the day it comes down to how the person using the technique can deliver it effectively. As Bruce Lee once said "I fear not the man who has practised 10000 kicks once, but I fear the man who has practiced one kick 10000 times."