Well ive done Kendo since I was about 4, but it really depends on what you identify with and what good clubs there are in your area, whatever you pick your better off doing something with a decent master. I would say Taekwondo, Wing Chun, karate are good places to start for self defense. Tai Chi is excellent but it will be 5-8 years before your truely capable of defending yourself with it which is too long I think.
Someone pointed out things like Karv Maga would basically give you jail time if you use them because they revolve around getting the kill. Any martial art will teach you to do that, but not from the off, control and discipline should be learnt before that IMO. So I wouldnt recommend those.
Boxing is fine and will teach you some basics which would be good enough for almost any situation, its also a great way to get fit, but a decent martial artist will flatten a boxer every time. Not to say im not recommending it but people seem to be talking it up too much for my liking.
I would also like to say I wouldnt recommend Kendo for self defense at all.
Hope thats of some help, basically choose what suits you and do some research on the masters in your area.
Hawker