The tongue and the facial muscles are used the most (provided you are not paralyzed, or suffer from a stroke). The heart is commonly used to answer this question, but it is also an organ.
diaphram
the diaphragm is the muscle(s) that are most important muscle used in inspiration (inhalation)?
Diaphragm
mouth muscleeeee (:
Your heart
The diaphragm, it is the main muscle used for breathing.
The myocardium, or heart, is the most used muscle in the body.
In the human body, the host used muscle would be the tongue, or facial muscle muscles (provided you are not paralyzed). The most used organ would be the heart.
The heart
an erection
Quads; the quadriceps.
Hockey probably gains the most muscle. In hockey, leg, chest. back, shoulder, and all your arms are used.