Here is my opinion:

• If you are into Java/Android development, you may want to learn Kotlin. It's a beautifully designed language and has really started to pick up steam after Google I/O '17.
• Machine Learning (and AI) is also hot these days. It's on my personal checklist.
• If you are into React, check out React Native and React VR.
• One may also want to dig into Go and Swift.
• ... And it's never too late to learn the foundation of web i.e. JavaScript.

Before answering, let me ask you a question first - best in what - Mobile Development, Web Development, Desktop Development, UI Development, Game Development, IoT, Low Level Development and Hardware Development?

As you see, founding the best tech skills really is based on personal preferences. For one it could be JavaScript with React.js and GraphQL, because he loves working on the client-side and UI part of a web application.

For another, it could be C++ with Unreal Engine, because he want to create that awesome indy game.

And for you, it could be totally different. So, what are your interests? :)

Checkout React, its great.