The ECMAScript 2015 Standard
The most notable feature of ECMAScript 2015 is a direly needed native module system that allows for properly structuring code. Furthermore, there's a plethora of useful language features like arrow functions, classes, and template strings. Finally, ECMAScript 2015 includes many smaller additions like block-scoped variables and
In sum, all ECMAScript 2015 features make for a vastly improved language, and we should start using those features today.
Using a Transpiler
Once you decide that ECMAScript 2015 — or whichever version your code is written in — is supported widely enough for your purposes, you can simply change the transpilation target to a higher language version and have your code constructs emitted unchanged. This way, you can rely on native implementations rather than compiler-generated, lowered code.
Integration with Build Tools
tl;dr: ECMAScript 2015 brings a lot of goodness. Start using it today.