Final answer:
The shoulder joint is the most movable joint in the body, which also makes it one of the least stable. It is a ball-and-socket joint allowing a large range of movement due to limited ligament support that prioritizes mobility over stability.
Step-by-step explanation:
The joint in the body that is one of the most movable and, consequently, one of the least stable is the shoulder joint. The shoulder joint is a ball-and-socket joint, which allows a very large range of motion. This type of joint features the rounded head of one bone fitting into a cup-like structure of another bone. In the case of the shoulder, the head of the humerus fits into the glenoid cavity of the scapula. The large range of movement in the shoulder joint comes at the expense of stability, making the joint inherently less stable than others that allow less motion, such as the carpal bones in the wrist.
Contrary to what might be intuitive, the more mobile a joint, the weaker it tends to be. This is due to the limited amount of ligament support which allows for a wide range of movement but also means the bones are less constrained and can more easily be dislocated. Movable joints such as the shoulder and hip are crucial for everyday activities, from lifting and throwing to walking and sitting.