General:
* Fake
* Forged
* Spurious
* Bogus
* Imitation
* Fraudulent
* Phony
* Sham
* Artificial
* Deceptive
* Fictitious
* False
More specific:
* Counterfeit money - Fake money, forged currency
* Counterfeit goods - Knock-offs, pirated goods
* Counterfeit signature - Forgery, imitation
The best synonym will depend on the specific context.