Fasted CardioPicture this: wake up, don't hit snooze, roll out of bed in your workout clothes and literally hit the ground running. Maybe drink a glass of water first if it's going to be toasty outside. It sounds mildly torturous, but I love it. Fasted cardio not only gets your workout completed early and wakes you up better than any cup of coffee, but has also been shown to burn more fat. Who doesn't want that?
HIITIf you're looking for serious calorie burn, hanging out on the elliptical while flipping through Vogue probably isn't going to cut it. Plus, who has time for that? Studies show that you can achieve more progress with 15 minutes of interval training that you can in an hour jogging on the treadmill. In addition to being super efficient, HIIT has many other benefits. Basically any type of cardio can be converted into HIIT - even the aforementioned elliptical.
One final note: don't try to mix the two. Running fasted sprints in the morning is a recipe for disaster (and muscle loss). Keep your fasted cardio at a steady level, and fuel your HIIT.
Here is an example of my typical HIIT workout: