The strongest material for garage shelving is metal. That doesn't mean that there aren't plastic shelving units that offer high weight capacities, but the absolute sturdiest shelving units -- ones capable of holding 1,500 to 2,000 pounds per shelf -- are made of metal. It's also worth noting that wheels are inherently less stable than flat feet, so units with wheels have much lower weight capacities than those that sit on the ground without moving.