Dominionmaster covered many of the styles I'd have posted. Krav Maga is undoubtedly the most practical, followed closely by his/her other choice- Jeet Kun Do. Boxing is an excellent art. Most people never look at the effectiveness of the defensive skills (this was one of Mike Tyson's biggest assets!!!!!!!), angles, as well as generating THE most power out of a punch. Karate does not teach power punching nearly as well!
I disagree with the assertion that it's the practitioner that "makes it deadly". Yes, the practitioner makes all the difference in the world in competitive fighting, while strength, heart, toughness, clearheadedness (BIGGEST one right there- lose you head in a fight and you're getting your ass kicked. In those ways the practitioner makes the difference. Given the wrong style, they'll never capitalize on their full talent.