Easiest counter to any round kick- MT or karate/tkd- is something anyone and everyone should be able to do.
Assume you and your opponent are both standing with a left lead.
My opponent throws a right round kick- his rear leg, power kick'n- in this instance I step my right leg forward and to my right so that I am walking both into my opponent and to his left- away from the kick. The left leg follows my step and just kicks solid into the opponent's standing leg; if he does not fall you should be in good position to follow with a right cross or elbow depending upon the distance.
This counter can be done against a round kick to the leg, midsection, or head. It shold be practiced against all of them.
This counter works out just as well under karate/tkd rules which often do not allow leg kicks but do permit groin shots. Simply aim the leg kick higher, and if you do end up taking him out at the inner thigh just say it was an honest mistake- you really meant to crush his penis. This counter/drill is something I picked up in MMA and have successfully applied to TKD.
It's a satisfying feeling to to knock a man over with very little effort while he's trying to boot you in the head.