General synonyms:
* Contrary
* Reverse
* Inverse
* Converse
* Antithetical
* Counterpart
* Polar opposite
* Antonym (specifically for words)
* Mirror image
Synonyms for specific contexts:
* Different: When referring to a difference in quality or characteristic.
* Contrary: When referring to a disagreement or opposing opinion.
* Reverse: When referring to a physical or metaphorical reversal.
* Inverse: When referring to a mathematical or logical opposite.
The best synonym to use will depend on the specific situation and the meaning you want to convey.