Which muscle is considered to be the "hardest working"?

The hardest working muscle is the heart. It pumps out 2 ounces of blood at every heartbeat. Daily the heart pumps at least 2,500 gallons of blood. The heart has the ability to beat over 3 billion times in a person’s life. Although most individuals have the same general set of muscles, there is some variability from one person to another. Generally, smooth muscles are not included with this total since most of these muscles are at cellular level and number in the billions. In terms of a cardiac muscle, we only have one of those- the heart.

Source: Library of Congress