muay thai works better in street fights and self defense, they act much faster than any martial arts in a stand-up close-range combat. the 2nd closest thing is boxing or karate.
the reason why muay thai is above boxing and karate is because muay thai uses the full body and specializes in precise and swift techniques, not much stance is required to initiate them which cuts down the time in which you need to land an attack from a neutral position.
i also advice against getting any ranks in martial arts, it only puts a burden in you when you use it for self defense, you may end up going to jail if you injure the assailant too heavily. the higher your rank in martial art, the longer you'd spend time in jail.
Muay Thai isn't very good in real fights, unless you're assuming you're fighting one on one on even odds with some other kid in the playground. And even if you were going to choose that sort of style, I'd suggest San Shou instead. It's like Muay Thai but with takedowns. For me, it's little more than a good workout.
I'd suggest Krav Maga. If you're just going to learn one thing, it's the most well rounded style you could learn for real world fighting scenarios. It encorporates several other martial arts (including Muay Thai), like Jujitsu... which personally, I'd recommend over Muay Thai anyways. Blocking, joint locks, throwing... Overall though, Aikido would be better. It's similar to jujitsu, but it's all about using your opponent's power against them, so for a small girl or small guy that would be fighting larger more powerful attackers, it's a very good style to learn.
Krav is like a self defense threat neutralization style though. If you really wanted to hurt someone, then you'd want to learn the one I practice, which is Long Ying (Dragon Style). The entire point of it is to cripple or kill your opponent. It makes a really good combination with Bak Mei (White Eyebrow), which uses much faster movement and strikes.
Above all though, if you're just getting into it for fighting purposes, go exclusively for Krav and get a concealed carry license and start carrying. Cause fighting only goes so far if you're against someone with a gun or a knife, or multiple assailants.
They say if you can fight of 5 assailants that's the theoretical upper limit that could attack you at any given time and you could beat any number of assailants, whether it's 5 or 5,000. But overall, you don't want to be in one of those situations to begin with... avoid it if at all possible.
