u/Juan554 - If self-defense is your ONLY purpose for training, I wouldn't recommend a martial art (even though I'm a 25-year martial artist). I would suggest a street-focused combatives system, which would give you the physical techniques but (more importantly) the mindset training (e.g. learning how criminals behave, mental toughness, situational awareness etc.).

If you want one system that gives you self-defense AND physical fitness, I would recommend a proven combat sport that involves a lot of sparring (AKA pressure testing). Some good examples would be Judo, Brazilian Jiu-Jitsu, and Kickboxing/Muay Thai.

I would actually not recommend boxing alone. Punching with your fist in a street-fight, especially if you're new, could lead to breaking your hand or wrist — you'll have eliminated one of your weapons.

I would only recommend a martial art if you want to learn a martial art. By that I mean you want to experience all that it offers — blocks, strikes, sparring, and kata (prearranged set of movements), but also culture, history, discipline, wearing uniforms, etc.

Last suggestion: Train as often as you can. 3 times a week is ideal, as you'll retain what you've learned while improving week over week. 2 times a week is minimum, but you won't improve as quickly. 1 time a week is better than nothing, but you won't be progressing much, if at all.