Pick your favorite (the area you tend to find yourself spending more time on) or the one most in demand, high paying, or secure (whichever matters to you). It

Pick your favorite (the area you tend to find yourself spending more time on) or the one most in demand, high paying, or secure (whichever matters to you). It seems Machine Learning / AI / Data Science (though maybe it's "maturing" after becoming such a hot field), mobile development, React + Redux, etc are what's in demand these days.