Well that’s easy! It’s the one you do, and keep doing.
You’ll hear all sorts of discussions about whether running is better than weights, if weights are better than cardio, if swimming is better than running, if cycling is better than everything else put together – ad infinitum. But the easy answer to all that is that just moving more, and sometimes getting out of breath, is the best idea of them all, even though they each tax slightly or significantly different energy systems and muscles.
In an ideal world we’d do a variety of exercise types so that we kept all the muscles working and got a good all-round level of functional fitness* but in reality that can be hard time and enjoyment wise.
If I was pushed for my most definitive answer I’d say anything that sees you moving your own body along dynamically against resistance entirely under your own power. That would include walking, running and aerobics/dance type classes.
Walking and running in particular use a movement pattern that is functional because we’re designed to move along on our feet. Exercising this way helps in all sort of life areas, it taxes the heart and lungs as well as providing resistance to the major muscle groups in the legs and bum. Whereas lifting a heavy barbell above the head tends not to translate in to real life functional activities unless we’re a hod carrier or something! But even with running and walking we can get weak up top, so adding a few press ups and core moves at the end of each session is a quick fix.
Find something you LIKE
But whatever you LIKE doing and will do often is fantastic! It’s important to find something you enjoy doing so you’ll carry on doing it. It’s all very well going to the gym for a month but if you don’t really like it, or it’s a kerfuffle, or you only joined because it was a new year, you won’t carry on. So even if it’s the best thing in the world it ain’t gonna be any good if you’re not there doing it!
Conclusion: The best thing is to move much more, and do it often/forever. Get out of breath and sweaty sometimes too!
*functional fitness is that which helps us to carry out daily and lifelong activities like carrying shopping, walking, lifting children or a chest of drawers, opening jars and so on ￼:-)