Which is the largest muscle in the human body?

Great question! The largest muscle in the human body would be the gluteus maximus, which is also one of the human body's strongest muscles. 
There are three gluteal muscles: the gluteus maximus, the gluteus medius, and the gluteus minimus. All of these make up the area of the body which we usually refer to as the buttocks. This trio of muscles is what people are talking about if you ever hear someone say they're working out their "glutes".
The gluteus maximus muscle is connected to multiple bones, and, along with the other gluteal muscles, is responsible for the body being able to move its legs and hips, as well as keep the trunk of the body in an upright position. The gluteus maximus helps you sit down, stand up, climb stairs, and so much more.