General Antonyms:
* Praise
* Commendation
* Encomium
* Flattery (though often insincere, it can be the opposite of harmful criticism)
* Truth (as calumny is a falsehood)
* Honesty
* Integrity
Specific Antonyms:
* Exoneration (clearing someone of blame)
* Justification (providing a valid reason for an action)
* vindication (proving someone innocent)
* Appreciation (showing gratitude or admiration)
* Respect
Informal Antonyms:
* Kudos
* Accolades
* Cheers
* High fives
The best antonym will depend on the specific context of the sentence or situation.