We all know cardiovascular work is important if you want a strong heart, lower body fat (in most cases), and it's a great way to keep up on your caloric expenditure. Everyone has their own interpretation of what the "best" cardio to do is to maximize fat loss; even though countless studies have shown that higher intensity trumps LSD (long slow distance) in most categories.
Take Treadmill Hill sprints for example, say we jack a treadmill up to a 15 degree angle and put the speed on 8.5 MPH, now say I told you to Sprint for 20 seconds then jump off and rest for 20 seconds and repeat this for 15-20 minutes. This is a high intensity treadmill interval that will get your heart rate rocking and not only will you burn a lot of calories during this session but you will also get something called and EPOC (excess post exercise oxygen consumption) — that basically means that since you worked so hard your body will be working extra hard to recover and repair itself for a day or two after the intense "cardio" has ended.
Now, say we tell another client to hit the treadmill for some cardio, but this time there is no hill and a much slower speed. Lets say the treadmill is flat (no incline) and we have them jog for 20 minutes at 6.0 MPH. They are both doing "cardio" to keep it simple, one is much more taxing to the body and harder to perform, while the other is a little easier (not saying its easy). They both will be benefitting by burning calories, increasing their blood flow and burning fat.
But heres the kicker: The "best" cardio is subjective from person to person. We all read the research and we know that certain types of "cardio" may be more beneficial and less damaging to the joints than others, but the cardio that's BEST is the cardio that the individual/client will actually do on a consistent basis!
I can tell people a certain way is best but if they hate it, they wont do it! If a client loves an hour on the elliptical and does it 3-4 days a week combined with resistance training, than this person will get good results because honestly, any training program that you're on will work if you work it!