Hi docs, can you tell me if some martial arts are better than others in terms of staying in shape?

Core: Programs that work on core strength and endurance

Depends: Not all martial art styes are for everyone. Some are harder on the body than others (hard vs soft styles). All styles help your cardio and to some extent your core. My recommendation is to visit the schools in your area till you find one that is a good "fit' for you. If you have arthritis, I would stay away from the "hard styles" (karate, boxing, grappling arts) and look into tai chi or kung fu