I'd personally love to see a frontend-heavy codebase that is using relatively new tooling (for example, ES2015 with Babel, React on a significant scale, etc.) if anyone would like to share :).
Derek recommends Backbone and Underscore - at least they can be found nicely annotated here:
I'd start with the repos for the libraries/framework(s) you've been using lately. Or for something a little different, take a look at Babel's codebase (https://github.com/babel/babel).
While it might bring a few boos or hisses these days, there are a lot of lessons to be learned from reading the jQuery code base.
Even better is the annotated version of the original jQuery release, which contains John's rationale about how things are the way they are.