Answer
The flywheel should be shaped like a hoop.
Work Step by Step
The moment of inertia of an object increases as mass is moved to be farther from the axis of rotation. For an object of a given size, the moment of inertia is maximized when all of the mass is as far from the axis as possible, i.e., the flywheel should be shaped like a hoop, with all of the mass at the rim (or as much as possible).
