If all the droplets were the same size, which would fall most slowly through the apparatus?

If all the droplets were the same size, the droplet with the smallest charge would fall most slowly through the apparatus. Basically, buoyancy is the same for all droplets but the electrical force would be different for each. The higher the electrical force the greater is the gravitational pull towards that droplet. Therefore, the droplet that has the smallest charge would drop most slowly.