The most efficient exercise is...

You may have made a commitment to get into better shape this year, but there's nothing that says you can't get it done efficiently, i.e., the least amount of time and sweat required.
Is there such an exercise, especially for a beginner?

YES

On the most recent podcast of Freakonomics, exercise experts unveil the holy grail of fitness improvement: the squat.
This stand-in-place-need-no-special-equipment movement is the one exercise that will pack the most health improvement for the time spent. Some of the benefits are:

Image from www.sweatlikeapig.com

• Maintain mobility. Being able to get in and out of a chair without help is a top determinant as to whether people will be able to live independently or have to move to assisted living or a nursing home.
• Increase functional strength. While you are building your leg muscles you are simultaneously increasing your body's nerve networks to work your muscles more efficiently all over.
• Tone your butt. Try it and see if you don't feel a burn in your butt.
• Improve your balance. Again, another characteristic of aging is that people easily lose their balance unless they work to maintain it. One of the largest risks to elderly people is falling. Committing to daily squats will help improve communication between your brain and body muscles.
• Increase upper body strength. How does a move that resembles getting in and out of chair work your arms? Squats stimulate your body to produce more anabolic hormones which get translated to bigger muscles all over.

Like any exercise, proper form is vital to prevent injury. But when done correctly, consistently, and frequently, the squat is one that will keep you on the move for many years to come.