Since I'm a fan of MMA (Mixed Martial Arts) and Pride fighting is what I love most, I would have to go with Brazilian Ju-Jitsu and Muay Thai! Here are my reasons:
In close quarter combat, you don't get the choreographed steps like most disciplines do, i.e. punch with the right hand, you block with this and that. Muay Thai is a natural instinctive combat style. You don't have the one two step... plus you're using your feet which is an advantage for reach.
Most of the time during a fight, you grab one another and you both fall flat on the ground. That's where BJJ come in handy. You learn to defend, attack and submit while on the ground, much like wrestling.
In any other discipline, you barely use the elbow and knees. Both of them have the hardest bone structure, thus the most effective weapon. Muay Thai uses both.
If the opponent is a boxer/standup, all you need is to take him down and do the damage on the ground. Take away his weapon (fist/feet) and you win the fight. (BJJ)
That's from my perspective. I've been watching UFC and Pride for a long time now and these two disciplines are the deadliest. Never seen a Kung-Fu fighter on UFC but I did see one video where a Kung-Fu master of 10+ years fought a Blackbelt Brazilian Ju-Jitsu (Royce Gracie, for all UFC and Pride fans) and the Kung-Fu master got beaten so bad they had to stop the fight. Royce Gracie choked him out.