Jump Spinning Back Kick but probably because it is the first jump spinning kick I learned how to do well. Okay, that and the amount of power I can get when I throw it.
I also like the twist kick because it isn't used all that often and is sneaky :ninja:
My favorite would have to be the roundhouse with the instep of the foot. I also really like the spinning back kick. Crescent kicks are really good too.
For kicks I'm capable of right now, the hook kick. Just something about that kick that I feel I can get more power out of than most of the others I can do.
Otherwise, the tornado kick, just for the "Oooooo!" factor.
On one hand, I agree with Kwanjang in that the spinning heal kick is just a beautiful way to put a hard piece of bone (the heel) on somebody's head with a whole lotta power. That goes along with an appreciation of the skill it takes to accomplish this.
On the OTHER hand,
I absolutely LOVE the front kick. Done correctly, ahp chagi can either knock someone on their butt OR penetrate and break stuff, all while keeping both arms in play for set up AND follow up punching.
I probably throw more front kicks while (continuous-style) sparring than any other kick.
And in self defense situations, it is easily thrown (don't need a lot of flexibility to throw it head high), fast and inconspicuous (don't have to put your body in any non-conventional positions: it is used from a stand-up position). You can fire out a front kick, do considerable damage, and if anyone blinks they will have completely missed what you've done.