Over the last four years, apartment managers in the biggest metros have inched rent growth back up to where it was prior to the downturn. But the momentum is beginning to lag compared to the leaps and bounds seen from 2010-2012.

The "sexy six" markets —New York, Los Angeles, San Francisco, Boston, Washington, D.C., and Seattle—have always been sound bets, though they too bottomed out in 2009 when rent growth hit a national low of -5.82 percent.

Washington, D.C., showed the least sexy rent growth numbers last year of the bunch, taking a step back for the first time since 2009. And rental gains in New York City are also starting to plateau. In fact, only two of the sexy six markets, Boston and Seattle, ended 2013 with higher rent growth numbers than the year before, according to data from Dallas-based Axiometrics.

Lindsay Machak is an Associate Editor for Multifamily Executive. Connect with her on Twitter @LMachak.