The best cardio exercise for any individual will vary with any number of factors including your level of fitness, your age, what you enjoy doing, the cost, etc.
However brisk walking is available to most people, doesn't cost much [just a pair of shoes] can be done a any level of fitness and can be integrated into most people's lifestyles. In addition, there are walking groups all over the country , if you don't want to do it alone.
Another good thing is that as you get fitter, you can fairly easily modify your exercise to match the "new you"
I can certainly walk much faster and further than I could when I started doing it and over rougher terraine. I can almost justify my very expensive hiking boots
Hana