I practice Manchuria Gung-Fu and it knocks the breath out of me most of the time. But, I suppose the original, unadulterated art forms practiced in the Shaolin temple are the purest and the most powerful. I'll hopefully make a journey to China sometime and see what the real thing is like.
I ve practiced Tae-kwon-do as a martial art and I think that two of the strongest martial arts are Muai-thai and Ninjutsu (Ninpo). Personally I prefer the second one. I would not rate any kind Kung fu amongst the strongest, but this is just my opinion.
The martial art isn't strong, YOU have to be strong, the stronger you are, the *stronger* your martial art will be. IT
when i say strong i mean the level of techniques, I'm not comparing individuals, just talking generally.