There are a variety of martial arts styles and each has its own fitness benefit. Some styles are strength-based, others are agility-based, others require speed and flexibility, and most require a combination of these. You might find that you are more suited to one style over another. On top of the physical aspect, each martial art also has its own philosophical focus. A good way to look at which martial arts style is to see how the arts' focus aligns with how you see the world. Find a club that offers multiple styles – or more specifically, find a club where the instructors cross train. You can ask them why they got into their practice, and see if that aligns with yours.