Loved this quiz. Surprisingly difficult! I daresay strictly speaking London ought to appear in both the eastern and western hemispheres, although that might make it a bit less interesting eh...
Paris should be on here. It's bigger than most of the cities in the 'western' section. It should be #7 or 8 depending on which version of the metropolitan area you use. But very embarrassing omission.
I was sitting with two minutes left, scanning through the US and Brazil, trying to work out what I had forgotten. Even considered West Africa, but for some reason Southwestern Europe didn't even cross my mind.