After much consideration the question concerning the strongest muscle in the human body has to be won by the muscle called, 'the heart'. Unlike other muscles that require rest for many hours the heart goes and goes and goes pumping millions of gallons of blood through the human body. Besides this, the strongest muscle in the human body is actually the one you sit on - the gluts or butt as it is commonly known.
The heart is not a muscle, it is an organ made up of several quite strong muscles. While it works all the time it is built for the long haul, as it is helped greatly by dynamic pressure and the efficiency of the resources at hand (Those muscles are fed by the most oxygen-rich blood, for starters). By density, however, the strongest muscle in the body is that inside the tongue, pound for pound.