Honestly, it is kind of futile asking here about the most stylish city. Hardly anyone has seen enough places to make that call. So people here will give you their favorites out of the narrow sample they have seen or based on some common stereotypes. This means you will get plenty of "New York" answers, because of the large group of Americans on this board. In addition there are many "London" suggestions, because everyone knows Savile Row and Jermyn Street. You will hear Milan and Tokyo and finally Hong Kong may be on the list, because people here know WW Chan who happens to be located in HK.
I will not say that any of these answers are wrong (even though my observation is that the average man in Paris is dressed better than the average Londoner despite the great makers and stores in London). What you might want to think about is, however, if you really wanna write the 100th report on men's style in New York or the 1000th article on the big names on Savile Row or shopping in Milan. Do rather something interesting - pick some cities which are a (little bit) less known despite being sophisticated and "stylish". Some examples in Europe could be Madrid, Barcelona, Monte Carlo, Naples, Florence, Vienna, Munich, Stockholm. Or look at what is going on in Shanghai, the former home of the 'red gang'. I am sure are many interesting places in other regions like Latin America, South Asia, etc.
Edited by AlexE - 7/29/11 at 2:47pm