If you use strict REST then best lang is PHP7+nginx with cache (http request -> memcache -> db) or better use doctrine2 ^^ (good configuration is slimframework.com)

If you want real-time flow then better to use NodeJS and socket.io or socketcluster.io :)

I don't believe you'll ever get a good answer from this kind of question. You'll just end up reading people's opinions.

You could detail more what you expect from the language...that could help. Like, you need performance? You like expressive languages? How do you feel about types? You wanna learn it fast?

Not necessarily for React unless you want isomorphic architecture

Golang > Node > Python > Ruby