In a street fight, where should you strike with a round kick?

I'm 16, and have done Taekwondo for 5 years, along with kickboxing, MMA, and a few styles of internal Kung fu. I do anything to avoid a fight, but am wondering where the best place to strike with round kick would be? It's the first kick I throw in sparring, no doubt. I sometimes just use punches (jab, straight, hook, uppercut) and that one kick. In taekwondo we were trained to strike at the ribs, but we all know that tkd is purely for competition. In kickboxing, my instructor taught me to direct the kick behind the leg, as he said it was a knock out spot. I think I would prefer this for a self defense strike, due to the fact it would land faster, and have much more power although I doubt it would be a knockout technique. But, I also like torso directed sidekicks, and train with them the most. So, what do you think would be most effective?