Looking at popularity using Web analytics and Google Trends data since 2004. for overall interest -- runners and online spectators -- the more revealing answer may come from a different perspective: via web analytics. When it comes to site visits and google searches on race day, the Boston Marathon is clearly the most popular, by a wide margin.
Google Trends data from 2004 to 2011 indicates interest in the Boston Marathon (blue line) is more than double relative web/search traffic of the New York City Marathon (green line). The Chicago Marathon (red line) lags both, but may catch up to New York City Marathon in 2012.
(While search and site visit traffic is high, it's a bit surprising to see a drop overall in 2007; we're not sure why).
Web analytics aside, The Boston Marathon, Chicago Marathon and New York City Marathon easily rank among the best and most popular marathons events. The web analytics data does not necessarily match popularity in terms of runners' interest and participation.
Interest in Participating. Interest in participating in each race is extremely high. All three sell out -- reaching the maximum number of entrants -- just days (Chicago), and even hours (Boston 2010) after registration began.
Number of Finishers. More than 100,000 runners finished all three events combined in 2012. In terms of number of runners, Chicago is 50% larger than Boston, and New York has roughly 100 percent more finishers. So you can say each race is extremely popular: sold out, maximum finishers each year.
But a quick recap shows that nearly the opposite is true when it comes to web searches and site visits. One wonders, if there were no limits on number of participants per race, if Boston would win the runners' most popular rank as well.