Exercise efficiency is linked to goal you are trying to accomplish. For cardiovascular workout use walking or running . For balance, strength & stretching I like yoga.
All exercise is good exercise. Beyond that, it depends on your budget, interests and goals. Ideally, whatever exercise you choose, you should aim for something cardiovascular (that gets your blood pumping, your body moving, and makes you slightly out of breath) and something muscle building (something that involves resistance or bearing weight). This takes care of your heart health and helps build and maintain a healthy amount of muscle – everything you need to be healthy. I love running because it gives me space and solitude, and, beyond the cost of a pair of trainers and comfortable clothing, it doesn't cost anything. I feel the same way about walking. Both can be done any time and anywhere, providing maximum flexibility to fit them into your schedule. Swimming is great as it provides cardiovascular and muscle strengthening into the same activity. I also have some hand weights for muscle strengthening at home. Getting into an exercise activity as a sport and then working to get better at the sport gives exercise purpose and goals, and is a way to make exercise sessions fun. If you can, find an instructor, if not, there are loads of apps and online resources which are free or low cost. Experiment and see what you enjoy – deriving enjoyment is what will driving you to push your trainers on and get out the front door and doing it.