The greatest sin against God is rebellion because it is part of every sin we sin.  We rebel against God and God only when we sin.  I think Psalm 51 says it best!

Psalm 51:1-4- Have mercy on me, O God, according to your abundant mercy blot out my transgressions.  Wash me thoroughly from my iniquity, and cleanse me from my sin!  For I know my transgressions, and my sin is ever before me.  Against you, you only, have I sinned and done what is evil in your sight, so that you may be justified in your words and blameless in your judgment.