The best cardio is the one you keep doing consistently.
From an exertion standpoint, IMO the one that requires the most work from your body will do it. I personally find the best results from the stepmill. My favorite is a treadmill because its easy for me to do for a couple hours if I need to. (Competition makes you do crazy things sometimes.) And I have a stairmaster at home because it is the only one that actually fits in my house. Thus it is the one I go to in the morning for AM cardio.
From an exertion standpoint, IMO the one that requires the most work from your body will do it. I personally find the best results from the stepmill. My favorite is a treadmill because its easy for me to do for a couple hours if I need to. (Competition makes you do crazy things sometimes.) And I have a stairmaster at home because it is the only one that actually fits in my house. Thus it is the one I go to in the morning for AM cardio.