It's all about heart rate.
Your calorie burn is determined by your heart rate, regardless of which cardio machine you use. For example, if your HR is 150 on the elliptical, 150 on the bike, and 150 on the treadmill, then you're burning the same number of calories per minute on all three machines (of course, you have to completely ignore the fairy tale calorie readouts on the machines!)
So the most effective cardio machine is the one that 1. you like to use (very important!) and 2. will elevate and sustain your heart rate to your target zone.
Nelie's right about ellipticals being very effective for calorie burning. Some studies have shown that the perceived effort to use the elliptical is less than other cardio machines -- in other words, it seems easier to elevate your HR on an elliptical than on other cardio machines. You don't feel like you're working as hard as you actually are, so you can do it longer and more intensely.
I almost exclusively use an elliptical for cardio because of my rotten knees. Adaptation to a cardio machine isn't nearly as much of a problem as is adaptation to an weightraining routine, so don't be too concerned about using the same machine(s) most of the time. So long as your HR is where you want it to be, then you're getting an effective workout. If your body adapts by getting more efficient, then all you have to do is kick up the resistance to bump up your HR.
Nelie's also right about HIIT. Not only will you burn max calories during your workout, but your metabolism will be elevated for hours afterwards and so your calories burned will be greater.
So bottom line ... your heart rate will tell you which is the most effective machine for you.