How can we reconcile that the heaviest thing in the balance is the declaration of "La ilaha illa Allah" (There is no god but Allah), with the Hadith: "There is nothing heavier in the balance than good character"?

Monotheism (Tawhid) will be the heaviest thing in a servant's scale on the Day of Judgment, as stated in the Hadith of the Card (Hadith al-Bitaqa). As for the Hadith of Abu Darda, "There is nothing heavier in the Balance than good character," its apparent meaning is not that it is superior to the word of Monotheism. Rather, it refers to the good character of the believing monotheistic servant. The preference for good character in this context is over other types of character or supererogatory acts of worship. Through good character, a servant can attain the rank of those who fast and pray (regularly). The benefit of supererogatory acts of worship may be lost due to bad character, whereas good character is beneficial even if supererogatory acts are few. Allah Almighty says: (O you who have believed, do not invalidate your charities with reminders [of it] and injury) [Quran 2:264].