What's the Greatest Teshuva (Repentance)?

On Rosh Hashanah you have to do Teshuva (repentance). What's the greatest Teshuva? There's one big sin of mankind that they have to fix on Rosh Hashana, and that's the failure to recognize "Hashem melech" (Hashem is King). There's nothing in the world. It's all a deception. Hashem is the only entity that exists in the world. — Shofar of Rosh Hashanah (#609)