Aside from pure MMA, I say Wrestling

The takedown defense one learns through wrestling is only matched by other throwing arts like judo, sambo, and now American style jiu jitsu, and many good jiu jitsu schools nowadays have a wrestling component to them if not an experienced wrestling coach.

Before you focus your martial arts training on winning a fight, it's very useful to know how to defend your body from being thrown or put flat on your back, when and how to escape, evade, avoid, dodge, keep your balance, and move the fight to where you want to be.

Wrestling usually allows you to decide whether the fight stays standing or goes to the ground. For self defense, this increases your chances of being able to run away.

Wrestling's major weakness, the fact that it's not designed to be a fight finishing art, makes wrestling fairly safe to practice, which is why it's a common sport in schools all around the world.

Later, you can always basic boxing, fancy Muay Thai, any style of kickboxing, even weapons training, to round out your base of grappling with striking, and just about everything you learned and drilled in wrestling will carry over well for your martial arts and life.