IMO there's no "best" exercise, the best way to get a great cardio is to mix up your workouts. If you always do the same exercise over and over again, your body will get used to it and you won't get any results.

The tabata protocol is great, it's high intensity interval training (increases VO2max, burns fat, gives endless cardio) . Perform once or twice a week.
8 intervals: 20 seconds all out intensity/ 10 seconds rest , then rest 1 mn (including the last rest) and do 4 or 5 more rounds if you're in good shape.
Don't forget to warm up thoroughly and don't do this if you have heart problems.

You can find a tabata clock easily on google.
You can do burpees, moutain climbers, bodyweight squats, sprawls, jump squats, tuck jumps, box jumps, plyo push ups, pull ups, chin ups, inverted rows, lunge jumps...

22 count burpees in sets of 10! I doubt anyone even knows how to do them! they are brutal