There are many words you could use to describe someone who is crazy, depending on the specific meaning you want to convey. Here are a few examples:
Formal:
* Insane: This is a medical term for someone who is mentally ill.
* Deranged: This describes someone who is mentally disturbed or disordered.
* Mentally ill: This is a general term for someone who has a mental illness.
Informal:
* Crazy: This is a common slang term for someone who is eccentric or unpredictable.
* Bonkers: This is a British slang term for someone who is crazy or mad.
* Loony: This is a slang term for someone who is eccentric or mentally unstable.
It's important to be mindful of the context and audience when choosing a word to describe someone as "crazy," as some terms can be considered offensive.