Don't know about the biggest. The Givi E52 Maxia is a good size being 52litre capacity, looks good is quite aerodynamic and can hold 2 full size helmets and still have room left.
You are limited with the weight you should carry plus being high up its not ideal to go for weighty items but good for bulky items. There are a lot of metal boxes that can be used too. I find the quick detachable plastic boxes suit me better. If I were off to Africa then I might change my mind. Round Europe the plastic ones I fell are better suited. What were you hoping to carry and for what use?