#1 Swim, #2 Run
If you have the means, you'll notice your bigger cardio gains from swimming. Swimming uses more muscles at one time than anything else.
#2 is running.
#3 would be a circuit - that forces you to work many muscles one after another with little or no rest.
In either case, if you want gains from cardio you cannot go easy, and you have to be consistent. Cardio gains are all about intensity; however, the irony is that you have gauge your intensity so you don't get injured.