Flutter or Kotlin and Swift or Java and Objective C?

Scala and Scala.js (https://www.scala-js.org/)

It's an amazing stack, integrates nicely into the existing ecosystem and has phenomenal usability.

It's all about your team and the ones that will inherit the project will want to use. So such thing as best.

But there is the surest bet - always the native stack. This gives you first class support for everything in the targeted platform and common ground that you can rely on other people to step on when they come in.