On current trends--with Atlanta stagnating (or slightly falling) and Dubai growing by double digits (15.2% in 2013)--expect the desert airport to fly past Hartsfield-Jackson in 2016 by my calculations. At its present rate of growth, Dubai will have 100 million passengers pass it by then. Atlanta has held the title of handling the most passengers since 1998, so a shake-up is due. Why Dubai? Again, it's a more convenient hub for travelers going between Europe and Asia:
KL is growing quickly too, but its growth is more regional--due to the proliferation of low-cost carriers--than international. Hence, I'd say Dubai has more upside potential:
Dubai International Airport gained 15.2 percent more passengers last year, for a total of 66.4 million people. The airline has worked to make its Middle East hub a more attractive connection point than Europe for travelers from Asia going to the West. It has also become one of the primary carriers connecting India and North America, given the Dubai hub’s geographic location and Emirates’ highly rated on-board service.
Emirates’ enormous growth was second only to the 19.1 percent advance in Kuala Lumpur, where two budget upstarts are battling the incumbent Malaysia Airlines with an enormous schedule of new flights and cheap fares.
