What Nobody said is true, any exercise is good. Although I cannot recommend just one exercise, you should find an exercise you enjoy doing, or at least one you are motivated to do. I hate swimming, so why would I force myself to go and swim? I wouldn't, I would choose an exercise I can stick to for quite some time. Also, don't "overuse" an exercise, switch up your program every now and then. Keep it fresh
Heck.. for me I walk 30mins, about 2 miles, and lift weights for 30mins, three times a week.. Then on the weekends I run 3 miles, three times.. The other two days per week are rest days. For me, it's been working great, and I feel great too!