I would say F#. It has most things you could want: - Proper functional programming support - Fast enough runtime - Cross-platform and open-source - Mainstre

I would say F#. It has most things you could want: - Proper functional programming support - Fast enough runtime - Cross-platform and open-source - Mainstream ecosystem - Large corporate backer - Compile to JS - Commercial and open-source tooling options … and yet C# is far more popular. This is because language adoption is driven by existing user base and other network effects NOT the quality of the language itself.