For pure cardio, the most popular thing - certainly in gyms - are the elliptical machines.

I'm personally a bigger fan of running on a treadmill because it's more natural than getting the feel for running on an elliptical. Although the lines are much longer for those machines, so I'm probably wrong.

The reason people prefer elliptical machines to treadmills is because it's gentler on your knees. But I don't think that running on a treadmill is that detrimental provided you stretch and aren't in danger of ligament damage.

Here's a nice article detailing some of the differences between the various machines: http://askthetrainer.com/best-cardio-machines.html
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I like spin classes the most.

Elliptical and the treadmill are good, but I think they're kind of boring (so is just regular bicycle).

But if you get a good teacher and someone who has a good taste in exercise music, then the right class can be a great workout.
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