No. 2: Seoul, South Korea
It may be a cliché to say, but Seoul's Metropolitan Subway has soul. With 19 lines and a clean, efficient demeanor, there's little reason to wonder why the system saw 2.52 billion passengers in 2011. Seoul's metro first opened in 1974, and as befits its Korean location, it's now kitted out with all the finest technology: mobile applications, massive electronic displays and, naturally, a "smart" payment system that will set you back 1,050 won, or about US$0.93, per ride.
Photo: Keith Lee