Well no martial art is perfect but you should find something you are into. I had terrible reflexes but I trained for years to improve them. Since your a swimmer something like boxing would fit you well. Just find a good trainer, someone who is not too aggressive. Boxing gets stamina up more than any martial art I have trained in and gives you good evasion and strikes. It will help your swimming as well. Outside of boxing you could try anything that seems interesting to you. I train boxing, judo, Brazillian jiu jitsu, Muy thai and I trained a little TKD and Krav. For the record BJJ is not better than Judo or jiu jitsu. It just focuses on the ground techniques only and incorporates moves illegal in Judo. O and you never want to get on the ground. In real fights awareness and mobility is key and on the ground you loose. BJJ or Judo trained wont matter on the ground because some punk will come kick you in the head or pull something and you will get messed up. Stay off the ground.

Your local dojo or gym from a well respected teacher.

Beware of mc-dojos, of which there are plenty.

Keep in mind also, that there is no "best" marital art. All martial arts have been proven to work. The problem in todays society is the watering down of martial arts to make things "user friendly".

It is up to the student to make what they learned effective.

90% of fights end up on the ground, so Jiu Jitsu is the best. Most MMA fighters study Brazilian style because it is better than Japanese style.

You don't have to be big to be good. I'm 200 lbs and I couldn't beat a 130 lb guy in my MMA class. We ended up having a stalemate.

go up to a big ghetto type thug and say "your momma is a snowblower" and then your dream will come true

I'll be honest I'm a chump and mostly talk. I want to learn how to fight or learn martial arts. But don't know where to start, I have terrible reaction and I'm 16. I'm fit from swimming so that's a plus. What kind of martial arts should I learn? I'm not that strong so I can't rely on brute strength. Maybe something that requires swiftness instead of strength idk. I'm just tired of being pushed around for being the youngest in my class... (Note I am not gonna take martial arts and start fights. I just want a defense mechanism) I already tried Kung fu, it's just I don't know how to react how do I fix that?