While there are all types of employment perks that are getting attention, from free food to on-site barbers, flexible work hours is by far the most appreciated gift an employer can extend to an employee. 

A survey of U.S. adults by Modis, a tech staffing firm, found that more than 50 percent chose flexible hours as the most important work perk. That came way ahead of unlimited vacation time, which came in second at 21 percent. Long-term paid parental leave came in a distant third, at only 8 percent. 

"People are happier when they are able to take care of things when they come up," Jack Cullen, president of Modis, told CNBC. "I think employers are recognizing this is real and (flexible hours) is a way to really attract and keep people."