There are several possible candidates for the strongest muscle in the body dependent on what set of criteria you are judging it on. Most people will tell you that either the heart, colonic, jaw or tongue muscles are the strongest, although they are all the strongest in their own right for performing different tasks that none of the others could do. Should we judge the strongest in terms of single feats of strength, or for having a constant workload, or even for how far the muscle can be pushed before it stops functioning properly are all questions that need to be addressed before a single definitive answer can be given in this case.
In terms of having the biggest workload then the heart is undoubtedly the strongest muscle as it has to work 24 hours per day, every day to keep us alive. It also allows all the other muscles to work by supplying them with oxygen and energy through the bloodstream. A lot of the other muscles we can live without or replace with artificial intervention, but the heart is still required by all of us to survive, and as well as working constantly it has to work harder at times when we are exerting ourselves or stressed for example. Because of this it is also one of the strongest in terms of endurance, and is used by fat the most times in our lifetimes.
As a single feat of power though the heart isn't that strong compared to some of the other muscles in the body. The jaw for example allows us to bite through many different tough food types that we may not be able to harm using our hands, and creates pressure in excess of a vice when we bite down on something as hard as we can. In fact the jaw muscles have the power to clamp so powerfully as to break all of our teeth in the process if you had any reason to do so.(i probably wouldn't try this one personally).
There are only a few animal species that can match humans for jaw strength, and a lot of them are purely carnivorous rather then omnivorous as we are, which requires them to have a strong bite to kill their prey. These species also tend to be a lot larger then the average human as well, such as the tiger or the polar bear.
Similarly muscles such as the diaphragm muscles, or the core muscles in the thighs are also very strong and can move the entire body themselves. Similarly to the heart, the muscles in the diaphragm have to move the lungs and ribs up and down all the time as well, so are in effect equally as important. Even when we are laying on our fronts these muscles are strong enough to move our entire bodies up enough to enable breathing to take place and for the lungs to inflate.
Even the often overlooked shoulder and arm muscles are capable of incredible feats. There have been many cases of people getting a sudden rush of adrenalin in an emergency situation and lifting cars to free trapped people for example, which would make them contenders as well. These muscles can also be developed to the largest extent making them larger and more powerful than they would normally be, in the case of weightlifters or athletes.
Overall this leads to the fact that determining the strongest muscle in the body is dependent on the set of criteria which you are using and that is is very subjective when trying to name a single muscle that is stronger than any others. Popular and even medical opinion is divided about this question and so there really is no clear answer.