This is a difficult question to answer without knowing more about the app you're building. There are tons of different platforms out there and the landscape is evolving rapidly. As a result, I don't believe anyone can claim to know all of them in detail, so a lot of it will come down to having a skilled development team.

I have personally built full stack solutions in a wide variety of domains, languages and platforms for the last 15 years or so. In my experience, there is no single "best programming language": many of the popular languages will be up to the task. Similarly, there is no single "best app framework", though one will lend itself better to a specific problem than the other. The key to success, IMHO, is having experienced developers that you can trust and rely upon to make an informed decision and then see it through.

Generally speaking, I tend to steer clear of cross-platform, "develop once, run anywhere" solutions, as that often solves for the greatest common denominator that doesn't feel quite right on any platform - exactly the opposite of what it takes to win in the extremely competitive mobile market.

I'd be happy to jump on a call and discuss your concrete situation. The more details you can share about the type of app you're building, the easier it will be for me to provide specific recommendations.