The longer I've been in tech and the more I learn, the less strong opinions I have. There are things you learn that should be done a certain way, but if the rationale behind that is strong/ubiquitous enough, it's not really an opinion any more. For example, I could say 'metrics/analytics/monitoring is super important... etc.' now the interviewer can decide to either agree with this fairly obvious/ubiquitous 'opinion', or decide this point is not interesting/controversial enough.
Ultimately, I think it's a terrible question - there's no 'right' answer, and you're completely at the mercy of the interviewer's subjective opinions/feelings.
If you want to hear about something the interviewee has passion/knowledge of - ask them about a project they worked on, what problems they ran into, what they learned, etc.