Is it coziest or cosiest?

It depends whether you are using American English or British English. The American English spelling of the superlative form of cozier is coziest, and in British English it is spelled cosiest.

This is related to the spelling of the root word cozy/cosy, meaning “warm, comforting, and snug.”

The QuillBot Grammar Checker has selectable language regions, so you can always be confident that you have spelled coziest/cosiest correctly.