For me it was like - when I was young I was in trouble many times. I had some judo - that mostly solved my teenage brawls. (It always ended on the ground by O Goshi and Kata Gatame 'til surrender.)
When I began to train serious martial arts I had a couple of encounters and I was always able to talk me out of it.
All that training to avoid fighting. Isn't that, what it is all about?
If you can't talk, (maybe you are robbed or it's just bad luck) something that requires not too much skill but is highly effective will serve best.
Your fine motor skills will be shut off when the adrenalin is flowing and your knees start to shiver. You need easy, agressive , gross mottorik skills. Palm strike to the side of the neck, the ear, mouth or nose. Pulling hair, kick to shin or knee.. And run...
Something easy to learn, good to grind in.
Sure - years of training - thousands and thousands of repetitions will help. But if if you want to be ready soon... Military style close combat, Combatives, Krav Maga. Fast and effective. (Check out Lee Morisson from Urban Combatives. Real stress situation training is key to self defence)
If you want to evolve and learn an art form. Go for BJJ, Boxing, traditional Muay Thai, Judo or Karate. Even Capoeira or Aikido are lots of fun and put your body and soul to another level. (I've been once kicked by my Capoeira Mestre to the shin and I thought a truck hit me)
If you want it all - go for MMA. (But be prepared of encounter some wild young men who watch too much tv with severe issues who just want to ground and pound.. Make sure it's a good gym with trainers who follow some security standards)
Personally, when it comes to infight and weapon defence (knife and sticks) I'd go for JKD, Escrima/Arnis and Silat. Those mechanics are quiet universal and a perfect match. And the more confident you are, the less you appear a victim, the less conflict you will encounter.
Si vis pacem, para bellum!
All that said - I never felt better than after a two hours Capoeira session. To me it's the last real spiritual martial art. Inside the Roda you are naked and exposed. You can't hide who or what you are. It is a very transforming experience.
And you'll be never in a better shape in your life. It is an complete art. Singing, dancing, playing/building instruments, fighting without even touching and still compete. It's a kind of magic. ( If salto mortale is not your thing - check out Capoeira Angola).
Take some Luta Livre or BJJ in addition and you'll be fine.