No, the biggest sin is Shirk, believing in the power of more than one God. Burning the Quran is pretty high up there though. Why? Are you planning on doing it? Obviously the biggest sin would be the only one that cannot be forgiven: to denounce God and his part in your salvation. If you tell him that you don't need him to get to heaven and that he should stay out of it, then you've committed the greatest sin of all.