Some things did change over the year, though. Notably, the collective number of units owned by apartment investors declined by 98,768 units to 2.9 million. On the flip side, the nation's top 25 managers saw their portfolios increase by 6,719 to a record high of 2.8 million.

Transaction volumes also show just how sought-after multifamily was among investors. Some $85 billion in apartment product changed hands over the past year, an increase of nearly 50% and the highest level of transaction activity barring the boom years of 2006 to 2007. In fact, last year's average cap rate of 6.1% was the same as the 2006 cap for multifamily assets.

The economic and geopolitical uncertainty after the Great Recession “led many to think apartment investment was still too risky,” says Mark Obrinsky, NMHC's chief economist and vice president of research. “But by year-end 2012, the real risk was getting in too late, not too early.”

As such, he continues, “Top firms looked to position themselves to meet the higher demand, resulting in increases in transaction volume and new apartment completions.” Despite the pickup in activity, the rankings “showed the kind of stability that characterizes mature industries.” The ranking was conoducted with Kingsley Associates.