If there would be one single capital of the world, which city should it be and why?

Here are some global city rankings for your thoughts:

Global Power City Index 2009 http://www.mori-m-foundation.or....
1. NY, 2. London, 3. Paris, 4. Tokyo, 5. Singapore, 6. Berlin, 7.Vienna, 8. Amsterdam 9. Zurich 10. Hong Kong

A.T. Kearney Global Cities Index  http://www.atkearney.com/images/...
1. NY, 2. London, 3. Tokyo, 4. Paris, 5. Hong Kong 6. Chicago, 7. LA, 8, Singapore, 9. Sydney, 10. Seoul

PwC Citie of Opportunities http://www.pwc.com/en_US/us/citi... 
1. NY, 2. London, 3. Tokyo, 4. Paris, 5. Beijing, 6. Hong Kong, 7. Singapore, 8. Seoul, 9. Shanghai, 10. Chicago

The 150 richest cities (metropolitan area) in the world by GDP in 2005http://www.citymayors.com/statis...
1. Tokyo, 2. NY, 3. LA, 4. Chicago, 5. Paris, 6. London, 7. Osaka, 8. Mexico City, 9. Philadelphia, 10. Washington DC
* Richest cities and urban areas in 2020 http://www.citymayors.com/statis...

World's largest urban areas Population in 2006 http://www.citymayors.com/statis...
1. Tokyo, 2. Mexico City, 3. Mumbai, 4. NY, 5. Sao Paulo, 6. Delhi, 7. Calcutta, 8. Jakarta 9. Buenos Aires, 10. Dhaka
* 2020 Population forecast http://www.citymayors.com/statis...

Global Financial Center Ranking http://www.cityoflondon.gov.uk/N...
1. NY/London 3. Hong Kong, 4. Singapore, 5. Tokyo, 6. Chicago, 7. Zurich, 8. Shenzhen, 9. Sydney, 10. Shanghai

Mega-Region Ranking by Richard Florida http://creativeclass.typepad.com...Page 31 - 1. Greater Tokyo, 2. Bos-NY-Wash. 3. Chicago-Pittsburgh, 4. Amsterdam-Brussels-Antwarp, 5. Osaka-Nagoya, 6. London-Leed-Chester, 7. Rome-Milan-Turin, 8. Charlotte-Atlanta., 9. Southern California, 10. Frankfurt-Gar