I've been doing martial arts since I was a little kid. Trained in Tae Kwon do, Tang so doo, kung fu, kenpo, wing chun and a few others...For the last 5 years I've been doing more Muay Thai, MMA and Brazilian Jiu-Jitsu. I'm a purple belt in BJJ and I've fought pro MMA for a few years. I'm also a part time bouncer.
In my opinion, I would definitely train muay thai and some jiu-jitsu or other grappling art.
TKD is great for fun, and has some great moves that can win once in a while, but some of the theory is a bit colorful and unreasonable for a street fight. In all honesty, it really depends on who is teaching. I learned TKD from a very hard style of TKD designed to break and destroy, but not every style is like that. Some try to teach elaborate or moves that work in theory without ever being put in to practice.
Muay Thai and Jiu-Jitsu are very practical for any type of combat situation. The power and technique associated with good muay thai is ideal for self defense. Jiu Jitsu is an excellent combination to this since most fights end up on the ground. Being on the ground is very dangerous on the street and being able to effectively defend yourself and quickly get back up to your feet is essential. Also, being able to put a guy to sleep with a choke is better for you from a legal standpoint, then tuning someone up with kicks and punches.
Trust me, you don't want assault charges on your record. They carry weight on them