What is the greatest sin? I understand that there is one sin that cannot be forgiven and that is to blaspheme the Holy Spirit.

But beyond this, what is the greatest sin that a person can commit? Either voluntarily or involuntarily?

I believe that we might be able to gain some insight into all other sin through the answer of this question.
The greatest commandment is given to us by Jesus.


  Matthew 22:36-40 NIV
  36 “Teacher, which is the greatest commandment in the Law?”
  
  37 Jesus replied: “‘Love the Lord your God with all your heart and
  with all your soul and with all your mind.’[a] 38 This is the first
  and greatest commandment. 39 And the second is like it: ‘Love your
  neighbor as yourself.’[b] 40 All the Law and the Prophets hang on
  these two commandments.”


I believe it'd follow that the greatest sin would be breaking this commandment.  So I would say acting in an unloving manner would be the greatest sin.  Being unloving towards God would be the greatest, followed by being unloving towards your neighbor.

Also, since the commandment is the action of loving, having a lack of love could also be considered a sin.

Putting sin in that light is convicting.