I agree that Insanity Program is intense.. It is as they advertise, pretty insane! It took me awhile to even be able to finish one of the workouts and I still can't quite keep up with Shaun T..P90x was a good series as well but the cardio was not very difficult.. if it is cardio you are looking for, Insanity is the way to go!
Also look out for MMA style conditioning workouts - Over the last couple of years conditioning has started to play a huge part in MMA enthusiasts training and as such a large number of programmes have been released to accommodate that.
There is no cardio workout that is most intense one, many can be done at intensity levels that are sustainable and far beyond... The human body is the limit not the type of exercise.
Just pick one (they are all the most intense ones)
- running
- swimming
- bicycling
- treadmill
- elliptical
- stair climber
- aerobics
- etc.
I hear hillsprints are hellaciously brutal. However, it's not the type of exercise that makes something intense, it's the intensity you bring to it. If you want to pay money, I'm sure the P90X stuff could have you close to vomiting - if you can't get yourself to that point on your own.