What is the strongest muscle in human body?

By strength: The quadriceps.

By size to power ratio: The masseter (jaw).

"In ordinary parlance, muscular "strength" usually refers to the ability to exert a force on an external object—for example, lifting a weight. By this definition, the masseter or jaw muscle is the strongest. The 1992 Guinness Book of Records records the achievement of a bite strength of 975 lbf (4337 N) for two seconds. What distinguishes the masseter is not anything special about the muscle itself, but its advantage in working against a much shorter lever arm than other muscles.

If "strength" refers to the force exerted by the muscle itself, e.g., on the place where it inserts into a bone, then the strongest muscles are those with the largest cross-sectional area at their belly. This is because the tension exerted by an individual skeletal (striated) muscle fiber does not vary much, either from muscle to muscle, or with length. Each fiber can exert a force on the order of 0.3 micronewton. By this definition, the strongest muscle of the body is usually said to be the quadriceps femoris or the gluteus maximus."

https://en.wikipedia.org/wiki/Muscle

Many different muscles can be characterised as being the “strongest”. Muscular strength usually refers to the ability to exert force on an external object. By this definition, the masseter or jaw muscle is the strongest. Its insertion on the mandible allows it to work as a short-armed lever. If strength refers to the force exerted by the muscle itself, then the strongest muscles are those with the largest cross-sectional area at their belly — the quadriceps femoris (in the thigh) or the gluteus maximus (in the buttocks).

"There is no one answer for this question since there are different ways to measure strength. There is absolute strength (maximum force), dynamic strength (repeated motions), elastic strength (exert force quickly), and strength endurance (withstand fatigue)."

The hardest working muscle is the heart.
The strongest muscle based on its weight is the masseter (jaw)
The muscle that can pull with the greatest force is the soleus.
http://www.loc.gov/rr/scitech/mysteries/muscles.html

Muscular "strength" usually refers to the ability to exert a force on an external object—for example, lifting a weight. By this definition, the masseter or jaw muscle is the strongest.

If "strength" refers to the force exerted by the muscle itself, e.g., on the place where it inserts into a bone, then the strongest muscles are those with the largest cross-sectional area. By this definition, the strongest muscle of the body is usually said to be the quadriceps femoris or the gluteus maximus.

A shorter muscle will be stronger "pound for pound" (i.e., by weight) than a longer muscle. The myometrial layer of the uterus may be the strongest muscle by weight in the human body.

The external muscles of the eye are conspicuously large and strong in relation to the small size and weight of the eyeball. It is frequently said that they are "the strongest muscles for the job they have to do" and are sometimes claimed to be "100 times stronger than they need to be."

The heart has a claim to being the muscle that performs the largest quantity of physical work in the course of a lifetime.
https://en.wikipedia.org/wiki/Muscle#The_.22strongest.22_human_muscle

It depends on what is meant by "strongest". The masseter muscle (jaw)is the strongest based on its weight.

http://www.loc.gov/rr/scitech/mysteries/muscles.html

While there is no "definitive" answer, it is commonly thought that relative to size, the tongue is the strongest muscle.

http://www.wisegeek.com/what-is-the-strongest-muscle-in-the-body.htm