[color=66ff00]Ok, from the top:
Karate: You got Shotokan and wado ryu, two different 'schools' for want of a better description. Karate attempts to utilise force to defend against force. It's reasonably effective if you've a decent build and can take some knocks. Lots of straight, hard punching and kicking.
Kickboxing: Split into two main groups, sport kickboxing and combat kickboxing. Combat can be very effective if you spend the time to condition yourself (shins especially) it's a pretty realistic fighting art. Knees, elbows, shins etc. expect to get hard nuts practicing it. Sport isn't taken as a serious martial art though a practitoner would be able to take on the majority of Joe "drunk 'n disorderly" Public.
Tae kwon do: Korean, as someone else mentioned. Extremely popular due to it's flashyness. Like kickboxing split into sport and combat styles. 70% emphasis on legs, 30% everything else. Flexability is key and if you have bad knees forget about it. Skilled poponenets of the art are the guys doing 540 roundhouse kicks, axe kicks and anything else you'd see Van Damme do in one of his movies. Schools are everywhere so it's easy to find one, especially at universities from what I've seen. Sport is akin to the kickboxing form, in the sport version if you turn from your opponent they stop attacking. How often does that happen in the street.
I'm 6 foot two, 9 and a half stone max., not particularly flexable but fast and reasonably agile. I can't use brute force as I'd get my butt handed to me so I use wing chun, a fast, compact form of kung fu. It relies on redirection of force to make the opponent go where you want them to. Pretty popular (most poular form of kung fu) and relatively fast to learn the basics. No kicks above the waist (generally) and a lot of concentration on handwork. Pretty darn effective when utilised by a reasonably skilled practitioner. You are unlikely to pull a muscle even if not warmed up hence it's good for streetfighting and general tussles.
I'm obviously biased as I think Wing Chun is the most effective form but if you want to learn general neck kicking skills then pretty much any competent teacher of any of the styles will be able to teach you good, basic defense. I'd try to stay away from 'hard' styles (using force against force, victor is usually the stronger specimen) like those you have mentioned but to be honest any of them will take time and dedication.
BTW this is a pretty simple description of all of the arts so I kinda ommitted some details though the basic points are good.
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