Without a doubt the obvious choice for the greatest sin in existence would seem to be murder--in its many forms. War, genocide, abortion, euthanasia, crime, terrorism, and any other form of murder seems to represent the greatest affront to Life Himself in Jesus Christ.
But could there be an almost equally affront to God in a rather insidious sin among us?
U.S. Genocide stats: http://www.abort73.com/abortion_facts/us_abortion_statistics/
Since God is Just, and life is purely a gift, murder (in all its forms--or formlessness?) is certainly the greatest affront to God's Justice. But God is also (mysteriously) revealed as Mercy in the Crucified and Risen Jesus Christ.
So what sin is the greatest affront to God's Mercy?
In a word (?) un-forgiveness. For, in paradox, a person can, theoretically, be forgiven even for the crime of murder. God forgives as witnessed by Jesus on the Cross: 'Father, forgive them for they know not what they do.' And humans have forgiven, as witnessed by the Quakers in Pennsylvania, a few years back, who forgave the mass murderer of their children.
So we return to the beginning of our consideration of the greatest sin. Maybe the rejection of the grace of forgiveness is the greatest sin--even greater than murder!
Unforgiveness as hindrance to prayer: http://www.jesus-is-savior.com/Books,%20Tracts%20&%20Preaching/Printed%20Books/Dr%20John%20Rice/Hindrances/h_03.htm
Consider the Cross. The very murder of God! Then consider the Mercy of God as God's greater Wisdom triumphing through that murder as Mercy! God's Justice would have been our condemnation, and that would have been Just--in sight of our sinful rejection of Him. And that is why Jesus died for our sins, though Himself innocent, as the perfect Human Sacrifice for our sin.
But God's Mercy prevailed!
So when we cannot, or will not, forgive, for whatever reason, we must ask ourselves, are we thwarting God's greater Mercy?