First, I think it's important to determine the purpose of "cardio" before selecting an exercise.

I am passionate about running, and while I am certainly aware of the physical benefits, they are secondary to the enjoyment of the experience and my mental well-being.

If one is looking to lose weight, for instance, as many people who say, "I need to do cardio" are trying to do, this is not the best approach (you won't burn enough calories), but one will glean other rewards from it.

Ultimately, the best type of cardio is the one you enjoy doing, so my list is about the one I enjoy.