Without hesitation I will tell you the squat rains king of all exercises. The large burst of testosterone and other hormones during squats, are the major reasons why squats and dead lifts are very important for overall muscle building. It’s the same reason why doing a heavy set of dumbbell presses is a more effective mass builder than a chest fly. Why? because the compound movement ( multi joint ) is going to release more testosterone and GH during the movement than a chest fly.
The large burst of anabolic hormones resulting from squats and dead lifts, allows your muscles to benefit from this release simultaneously. Squats and dead lifts work many muscles simultaneously ( multi joint exercise ) in the body and will allow your body to receive stimulation from the release of the hormones during the exercise. The large amounts of testosterone and GH released from squats, is very crucial for being a catalyst for your muscle growth. Very little will be released in the smaller compound exercises, such as a bicep-curl. Therefore doing single joint exercises do not get the same bursts of GH and test as will having squats in your routine will.
If I was told I could only perform one movement to develop my overall physique and build strengh, it would be the squat.