Oh, London, most definitely.
But with some provisos:
1. At as large a resolution as possible.
2. But then make it as naturalistic as possible as well. Although I have never lived in London, I have visited it qauite a few times and know the layout of its streets, parks and river quite well, and it would bother me if its CiM version looked significantly different. I'd rather cut out some of the suburbs rather than distort the look of inner London.
3. Have all modes of transport - London had trams and trolleybuses in its day, and there is no reason why it could not have a monorail. Ideally, commuter railway lines running along railways and placeable by the player should be added as an option, since London has a lot of them.
4. Ideally, as well, driving on the left should be added as an option to the game. In fact, I don't understand why this was not done in the first place. I am not a professional programmer, but I think that it should not be particularly difficult to define "driving side" as a toggle-like parameter at the very top of the game parameters, and program all movement functions based on that. Everything is symmetrical - where you turn left in one option, you turn right in the other.