A cylinder has the least moment of inertia compared to a ring due to its mass distribution being closer to the axis of rotation. The moment of inertia (MOI) quantifies how mass is distributed relative to an axis, affecting an object's resistance to rotational motion. For a cylinder, more mass is concentrated near the center, leading to a lower MOI. In contrast, a ring has a greater moment of inertia because its mass is distributed further from the axis. Understanding these principles is essential for analyzing rotational dynamics in physics.