What is the best form of cardio?

The single most asked question I get is: what form of cardio exercise is the best?
The answer is actually really simple. All of them. If it gets your heart rate up, then it is good. However, different sports and activities may require a greater focus on one type.
For example (a simple example), if you are going to run a 10k, then you should probably consider running; if you are going to ride in a century race, then getting on a bicycle is what you should be doing; martial artists should consider jumping rope, if you have bad knees (knees that hurt, not knees that misbehave), then an elliptical may be the best form of cardio; for a tri-athlete, whatever your worst event is, that is the one you should put the most energy in to.

For general fitness; pick one form of cardio and do it for a few weeks, then pick another and do that for a few weeks. Repeat process. The idea is to not let your body get too used to nay one form of exercise. Mix it up and have fun!