Very tempting question. I can't stop with one. I will share all the code bases I want to explore and the reason behind it. I am lucky, as some code bases in my list are open sourced.
Digital Ocean - One of the cleanest products with perfection in every inch of its design. I just love using this product and I am sure a lot of developers too ! A clean product means a clean architecture. A clean architecture means clean code. So, I am sure that I will learn a bit of what simplicity really is by going through their code base.
Gitlab ( Open Sourced ) - Fast growing software development platform. These guys are giving a tough competition for its competitors Github. The reason why I want to explore gitlab instead of github is:
Telegram - Favorite messenger. Fast and secure. I am sure that lot of cool things like MTProto are being done in their code base. I would like to spend time reading about those architectures and their code implementations. And from their API docs, I guess that their backend service contains functional programming language ( most probably erlang :: correct me, if i am wrong here). It would be nice to read the source of an app that is written with functional language and running at this scale.
Some companies, have open sourced some parts of their codebases. Here are somethings, that I want to spend time in going through.
Hashicorp - DevOps Delivered, the right way. Industry leaders with amazing tools that are open sourced.
Square - Square played major role in teaching me how code can be written cleanly. They have excellent libraries written in java and made me to realize, how code in production can be written. My personal favorite is retrofit
Docker - Have gone through some parts of their codebase as part of my day job. Things really are straight forward here. I would like to take time and get a full picture of things going on here !
and many more.