Costume crazes come and go âlike when half your sixth grade class dressed up as a Harry Potter character for Halloween. According to HalloweenCostumes.com and the National Retail Federation, these are the most popular costumes from 1990 to today. Click through to find what everyone was dressing up as the year you were born. 2015: Harley Quinn from Suicide Squad. 2015: Harley Quinn from Suicide Squad.