According to me, adultery is the biggest sin of all.
Adultery means consensual sexual intercourse between a married person and a person other than the spouse.
Being married to someone is an auspicious thing in life and being loyal to them is the stepping stone of it. By doing adultery, a person breaks everything important in his like their marriage and their soul. As nobody want to admit to themselves that they were not loyal to the person whom they married and what good a person really is if he is not loyal to someone with whom he promised to share his life forever.