The biggest sin?

When I am in Bible Studies and a discussion of sin comes up, usually the "biggest sins" brought up are:

• Abortion
• Gay Sex
• No prayers in public schools (which isn't accurate.  You can pray in public school, you just can't have a state religion.  So, any religious group can pray).
• Disobeying authority (this one has political overtones depending on who is President)
• Any kind of talk about equality (you are stuck with your lot, wanting more or equitability causes "problems").
• Unbelief

Most of the times in these studies, it seems that there are some members who are very knowledgeable about the Bible.  That's why it is odd that Jesus words to the Pharisees from Matthew 22:37-40 are rarely brought up.  This was when the Pharisees asked Jesus what was the Greatest Commandment and he replied:

"Love the Lord your God with all your heart and all your soul and all your mind.  This is the first and Greatest Commandment.  And the second is like it: 'Love your neighbor as yourself.'  All the Law and Prophets hang on these first two commandments."

When I think of the biggest sin, I think it would be something that contradicted the Greatest Commandments of which Jesus spoke.  Are we doing all we can to follow these commandments?  Quite often, the list above is used in a manner that is unloving and divisive, which according to the Bible is not Christian and further drives those who may be seeking away from Christ.  Or it may just give more ammunition to those who don't want to believe.