Which circuit would have the brightest light?

If you connect bulbs in parallel across a given voltage supply (eg household mains supply) then they will all draw their rated current and will glow at their rated brightness. If you connect the same bulbs in series then they will glow very much less brightly. This is because the resistance of a bulb's element is also related to the power dissipated in the element.

A bulb's element is rated for operation at a particular voltage. You can certainly connect two 6v motorcycle bulbs in series, and connect them to a car battery at 12v, and the two bulbs will be just as bright as they were when individually operated from the motorcycle 6v batttery. However if you connect two 6v bulbs in parallel and then wire them to a 12v car battery they will only glow brighter for a fraction of a second and then ... pop. You will have overheated the elements beyond their designed power handling capability.