most popular adult Halloween costumes sold from 2009 to 2013.
Zombies have grown in popularity while clowns have fallen.
Witches, however, have topped the charts five years in a row. And given their dominance in the rankings, it's entirely possible they've been the most popular costume for much longer. (Although witch costumes certainly were unpopular in Salem back in, say, the late 1600s...)
Note: The rankings do not include all costumes, just those sold in retail stores and reported by the National Retail Federation.