Chelsea FC - UFC is a BRAND, not a martial art. There are many fighters in there who practice both Karate, Taekwondo and others.
But to answer your question Jerome; There is no 'best martial art', only good martial artists.
But, having said that. Taekwon-do is split into two dominant factions, Olympic Taekwondo and True Taekwon-do.
True Taekwon-do practises on the fundamental and more dangerous aspect of the sport, using strikes to end a fight as quick as possible. This is the same version pioneered by the great General Choi Hong Hi in April 1955.
Some disputes led to the breakaway and formation of the WTF (World Taekwondo Federation) and the ITF (International Taekwon-do Federation).
Unfortunately, the leader of the WTF was also a leading IOC member in Asia, and pulled strings to have it included in the Olympic games. But the ITF (the style i have practiced for over 10 years), is the original and superior martial art. The Taekwondo you see on the television is fake, useless and relies purely on entertainment...
As for Karate, well, again... There are many styles, Shotokon, Wado Ryu, Kenpo, Kyukushin etc... ut again, there are no dominant styles.
As for boxing. Well put it this way, my legs are longer than your arms, and i have NEVER lost to a boxer when i've been allowed to use my leg attacks.
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Here is ITF, TRUE Taekwon-do in action;
http://www.youtube.com/watch?v=Bd-9cw6-vrM&feature=related