Can I vote for all of them :)

I picked Generators because they are the only true addition to the language and add some serious value to the functional nature of javascript. We can support something similar in ES5, but not really...especially not without storing state ourselves. As for the rest:

Modules: Native module support is a simple, but welcome, addition! However, unlike generators, we already had this capability. This just gives us a single standard to live by.

Arrow Functions: syntax sugar, using macros or snippets works just as good, if not better.

Classes: Honest, the biggest problem I have with this is that it is damaging/lying. These aren't classes...it's just an alias to the prototype system that already exists in ES5 with a nicer definition syntax.

voted for generators because they are interesting but have horrible syntax and documentation :) messing around with generators is fun, though.

using arrow functions the most. because as an ex-ruby'ist and coffee script guy, i know and love fat arrows :)