The one you aren't using.
The body adapts very quickly to any kind of workout routine, whether it's your resitance training program or your cardio program. You should have variation in both at regular intervals.
So if you've been spending most of your cardio time on the same machine for over 6 weeks, it's time to change it up. Your body has developed a mechanical efficiency to that movement. Now if you are a marathon runner and you develop a mechanical efficiency to running, that is a good thing – it means you can run longer with less effort. If you are trying to change your body composition it is a bad thing – you get burn less calories, the workout is less intense, and you get less of an "afterburn" from the same amount of running.
In other words, the longer you do what you are doing, the less and less you get out of it. That's why I run into so many people who have been doing the same cardio routine and the same resistance training routine for a year or more, and haven't gotten any results in months and months. They have adapted. You need to work out smarter than that. One of my old trainers had a shirt that said "No Brain, No Gain."
My recommendation is to switch cardio machines. Switch from what ever you are using now to the least mechanically similar cardio machine. For example, in my gym: Switch from the treadmill, treadclimber , or eliptical to the rowing machine. Then three weeks from now switch from the rowing machine to swimming. And three weeks from then switch from swimming back to the treadmill or eliptical.
Always remember – the body responds extremely well to cycles. Not like "bicicles" I mean cycles of training. All in the world are cycles – day/night, the seasons, ect. Your workouts should move in cycles also.
Also check out my article: Your Cardio Routine Sucks, and Josh Hillis Cardio Rules
And if you are a runner: Weight Training and Cross Training for Runners: Myth vs. Fact
By Josh Hillis
Author of How To Lose The Stubborn Seven Pounds: Take Your Body from Good to Rockstar.
National Academy of Sports Medicine Certified Personal Trainer (NASM-CPT) and Performance Enhancement Specialist (NASM-PES)
Russian Kettlebell Challenge Certified Instructor (RKC) and Combat Applications Specialist (RKC/CAS)