Maybe worth updating this quiz? Wikipedia's current list of metro ridership has a few entries that didn't appear when this quiz was written in 2012, plus a few placing have changed. Which leads to another quiz idea - Metros that have had the Biggest Ridership Gains in the last 3 years :)
Definitely worth an update. Particularly in China the construction of new lines and stations has been explosive in recent times. I'd say the figures have changed a lot since then.
World's Busiest Commuter Railways Quiz