Sugar, especially in the US where it’s literally added into everything. What’s worse is the alternative (substitutes like aspartame) might also be a candidate and we just don’t know it yet because enough time hasn’t passed to study the long term effects. I try to take stevia as much as possible because it’s more “natural”, but only a few sugar-free products use it over aspartame. I read recently the WHO still considers aspartame as a carcinogen, but only in excessive amounts, like several glasses of soda a day.