Hopping on the elliptical five times a week, every week, at a level six isn't bad for you. But that's just because it's not really doing anything.
First of all, for your fitness regimen to produce results, you can't just be a slave to one exercise -- you have to keep your body guessing. "Your body is a smart machine and continuously adapts to the stress you put on it," explains NYC-based personal trainer Eric Mannarino. "To get the most out of any workout, you'll need to continuously perform movements that you're not accustomed to doing, otherwise you'll lose the effects of training."
On top of that, in order to torch calories, get stronger, and lean out, you need to incorporate exercises that actually challenge you in different ways, target different muscle groups, and incorporate both cardio and strength training.
To get you started (and me, because I'm filing for a trial separation from the treadmill), I asked personal trainers and fitness professionals with various focuses in the industry to declare what they deem the most efficient workouts out there. Then it's time to shake things up and integrate each exercise into your routine -- just have the sweat towel and Aleve ready to go.