I like incline sprint intervals the best.
The reason is it increases your V02 max the best, and greatly increases your explosive running power for any sport. As far as fat loss, if you read studies on hormones, it encourages fat loss without and loss of muscle (unlike long distance running). Plus, unlike intervals on bike, a sprint is something where it is quite easy to exert yourself at 100%.
My strong advice though is to get a heart rate monitor when you exercise so if your doing intervals you can verify that you are really pushing yourself. This won't be necessary if your going to do slower types of cardio (e.g., walking)
Unfortunately, it is quite taxing on the knees and has strained my knees in conjunction with squatting, jump squats and tennis (people who say long distance on the running is more taxing on the knees does not equate with my experience).