Originally Posted by
gnaget
Well, it doesn't really make sense to me to back track from Shibuya. Also Shibuya's NEX platform is next to New South Exit. If you exit there (easy and not crowded) then there's no official cab rank.
My only reservation is that Tokyo Station is very complex and the OP might spend a long time finding his/her way to a taxi rank.
N'EX platforms are marked "(B5) Sobu Line Platforms (Basement Level 5)" at the bottom of
this map and the best place to get a taxi for Tokyo Midtown will be the Marunouchi Exit of the station, which is at least on the right side of the station complex.