• From the time I started tracking the Z-Yen rankings in 2008, until September 2013, London was ranked No.1, with New York running a very close second. Typically, there was less than a 1% difference in the ratings between London and New York during that 5 year period.
At least once during that period (in March 2010), London and New York shared the No. 1 spot.
• From March 2014 until September 2015, New York took over the No. 1 position, by a small margin, with London hot on New York’s heels.
• Then, from September 2015 until September 2018, London regained the top spot.
• Now (September 2018) New York is back on top by the thinnest of margins – a rating of 788 for New York vs. 786 for London.
Note that these ratings and rankings, compiled and published every March and September, include a variety of factors – financial, economic, social, and life-style – in judging the desirability of major cities as places to live and do business.
The BIS Triennial Survey which @forex.com mentioned, on the other hand, considers only one metric – average daily foreign exchange turnover (forex volume).
For anyone interested in the current (September 2018) Z-Yen Global Financial Centres Index (note the British spelling ), here’s a link – Z-Yen (24) - Global Financial Centers Index - 24.pdf (3.5 MB)
And for anyone who wants to dig into the latest (September 2016) BIS Triennial Central Bank Survey, here it is – BIS Triennial Survey 2016.pdf (558.5 KB)