General Antonyms:
* Criticism
* Condemnation
* Reproach
* Blame
* Censure
* Rebuke
* Scolding
* Denunciation
* Obloquy
* Disparagement
* Contempt
* Scorn
More Specific Antonyms:
* Faultfinding (criticizing for flaws)
* Ridicule (making fun of)
* Insult (being rude or offensive)
* Denigration (attempting to lower someone's reputation)
* Vilification (using harsh or abusive language)
The best antonym will depend on the specific context in which you are using the word "praise."