General:
* Counterfeit
* Spurious
* Bogus
* Phony
* Artificial
* Imitation
* Sham
* Deceptive
* Fraudulent
More formal:
* Fictitious
* Fabricated
* Forged
* Counterfeit (can be formal)
* Adulterated
Informal:
* Fake (most common)
* Phony (often used with a negative connotation)
* Knock-off (typically refers to products)
* Poser (refers to someone pretending to be something they are not)
* Faux (usually used in a specific context, like "faux leather")
The best word to use will depend on the specific situation and the desired tone.