* Illegal: This is the most straightforward opposite, meaning something that is against the law.
* Unlawful: Similar to illegal, this emphasizes the violation of laws.
* Illicit: This implies something that is forbidden or secretive, often involving illegal activities.
* Unjust: This refers to something that is morally wrong, even if not necessarily illegal.
* Unconstitutional: This refers to something that violates the constitution of a country.
* Improper: This is a more general term for something that is not acceptable or appropriate, even if not explicitly illegal.
* Forbidden: This indicates something that is prohibited, often by a rule or regulation.
The best opposite for "legal" will depend on the specific situation and what you're trying to express.