Informal:
* Bonkers: This is a lighthearted and playful synonym for crazy.
* Loony: This is another informal synonym, often used to describe someone who is eccentric or outlandish.
* Nuts: This is a common slang term for crazy.
* Off the wall: This phrase describes someone or something that is unusual or eccentric.
* Wacky: This word suggests something that is silly or funny.
Formal:
* Insane: This word is more formal and clinical, often used to describe someone who is mentally ill.
* Deranged: This word suggests someone who is mentally disturbed or disordered.
* Unhinged: This word implies that someone is unstable or erratic.
* Mad: This is a more formal and literary synonym for crazy.
* Eccentric: This word describes someone who is unusual or unconventional.
Other:
* Absurd: This word suggests something that is ridiculous or illogical.
* Unreasonable: This word describes something that is not logical or sensible.
* Outrageous: This word implies something that is shocking or extreme.
The best synonym for "crazy" will depend on the specific situation and the tone you want to convey.