Let’s consider the butterfly. One of the most taxing movements in sports, the butterfly requires greater energy than bicycling at 14 miles per hour, running a 10-minute mile, playing competitive basketball or carrying furniture upstairs. It burns more calories, demands larger doses of oxygen and elicits more fatigue than those other activities, meaning that over time it should increase a swimmer’s endurance and contribute to weight control.

Ask a dozen physiologists which exercise is best, and you’ll get a dozen wildly divergent replies. “Trying to choose” a single best exercise is “like trying to condense the entire field” of exercise science, said Martin Gibala, the chairman of the department of kinesiology at McMaster University in Hamilton, Ontario.

Fat Thanks to Sunita K. for the tip!

What’s the Best Exercise? – NYTimes.com