Elliptical

Mine has a "fat burn" mode (NordicTrak elliptical from a few years ago). I generally do a 45 min workout of:

5 min warmup. 15 mins of going between level 2-3, 10 mins of level 4-5 alternating, 10 mins of level 5-6 alternating, 5 min cool down. Usually I get my heart rate up to 60% within 5-10 mins and rises through out the workout. By the end of the work out I'm at 80%. I use a HRM to pace myself. Even though It's a "fat burn" mode I'm at my aerobic threshold (60-80%) for most of the workout.

In the end the best way to burn the most calories is to keep yourself in the aerobic range for as much as possible and not to push yourself over 80% or you'll fatigue early.

I've experimented and Fat Burn burns the most, at a higher elevation and incline. I go as high as I can maintain for 45 min to an hour.

I tried something new today. On my computer I had a playlist of the songs I liked to exercise to...started with warm up tunes, followed by fast ones for an aerobic work out and ending with some slow and mellow ones for cool down. Well I transferred the lot of them to my I pod and played them on "shuffle'. (Random order.) I found it really motivating and burned about 100 calories more than usual probably because when I hit the fast ones I was more energised and made the most of them. My method wasn't very scientific but I think it approximated the high intensity interval training that they say is particularly effective. See for example:
http://www.builtlean.com/2010/06/04/high-intensity-interval-training-hiit-best-cardio-to-burn-fat/