According to most online sources, the gluteus maximus or buttock muscle is the biggest of the 639 named muscles in the human body.
The longest muscle in the human body is the sartorius. This narrow muscle of the thigh passes obliquely across the front of the thigh and helps rotate the leg to the position assumed in sitting cross-legged.
The strongest muscle depends on how you measure strength. A good case could be made for the heart!