It's unimportant to me whether TKD would be effective in mma or a street fight. I do it for the conditioning, which is excellent, and the speed, balance and agility I mentioned before. someone said it's all in how you train, which I agree with wholeheartedly.
Having said that, i would take judo before muay thai if I had the opportunity to take an aggression-based as opposed to a self-defense art. because in judo you learn techniques that can circumvent if yr. a lot smaller than your opponent. all my sparring partners are 4-6 inches taller than me with a vastly longer reach, longer legs and heavy hands, and getting hit by them is a useful dose of what i might (hopefully not) experience in the real world.