Therefore the one who delivered Me to you has the greater sin. John 19:11

Sin is sin before God, right?
Personally I see murder a more horrible sin than lying. The Bible declares the wage for either is death (Rom 6:23).
Yet, Jesus told Pilate that Judas, the Sanhedrin, and High Priests – all of whom had a hand in delivering Jesus to him, committed a greater sin.

The Bible offers other examples of great sin.
Moses approached God and said, “Oh, these people have committed a great sin, and have made for themselves a god of gold!” Ex 32:31

Old Testament priest named Eli had two sons that took meat from the sacrifice for their own.
Therefore the sin of the young men was very great before the Lord, for men abhorred the offering of the Lord. 1 Sam 2:17

Jeroboam drove Israel from following the Lord, and made them commit a great sin (2 Kings 17:21) when he placed idols around the land to keep people from going to Jerusalem to worship.

It seems to me the greatest sin is failing to recognize and worship God for who He is.
All sin is forgivable except one – to him who blasphemes against the Holy Spirit, it will not be forgiven. Luke 12:10
The greatest sin is lack of faith and unbelief. This is unforgivable because the unbelieving person will not ask for forgiveness since he/she does not believe.