The best successes I've worked with have been
1. A keto recipe app with hardcoded recipes made in two weeks. Users told me the way I displayed it was confusing and that it needed carb counts. I did the fixes and they started throwing money at me.
2. A social media app for complaints. Turns out people wanted social media for a small area, e.g. everyone in an apartment area. It ended up being a visitor management/appointment kind of app with social media features.