Anything that's going to embarrass you
We all have embarrassing inquiries, but if you're concerned they could be used against you, say, in a political race, then it's best not to search Google for it! Even if you permanently delete your search, you're out of luck according to security experts Svea Eckert and Andreas Dewes, who managed to gather the online habits of some 3,000,000 German citizens without their knowledge, as well as the private web history of prominent public figures, including judges and politicians. Besides that, you really don't want ads popping up for those embarrassing inquiries, especially if your boss is lurking over your shoulder.
9 Things You Should Never Google, Source:https://www.rd.com/list/things-never-google-search/