Easily it is the unlimited PTO. Obviously I try not to go crazy with it, but it' great that I can take a sick day if I need it and a travel day when I want it. Free lunch would be my other answer but it could use some improvement or a new caterer.
Personally, the flexibility to work from home/unlimited vacation days is huge for me. I feel even more motivated to use my time wisely and find myself consistently producing high-quality work and I attribute a lot of that to leadership creating a great environment of work/life balance.
Lower than market Value by a significant amount