Formal:
* Liar: This is the most direct and common term.
* Deceiver: Emphasizes the act of misleading someone.
* Fraud: Implies an intention to gain something by lying.
* Perjurer: Specifically refers to someone who lies under oath.
Informal:
* Fibber: Suggests someone who tells small, harmless lies.
* Prevaricator: A more sophisticated word for someone who avoids telling the truth.
* Phony: Implies that someone is insincere and dishonest.
* Con artist: Someone who uses deception for personal gain.
Figurative:
* Wolf in sheep's clothing: Someone who appears harmless but is actually deceitful.
* Snake in the grass: Someone who is secretly treacherous.
The best word to use depends on the context and the level of seriousness of the lies.