I worked for several years at Ankama on the game client of the MMORPG Dofus. When I arrived, the code base had already undergone an entire refactor and a change from ActionScript 2 to 3. At that time the developers had spent some time to breakdown the code into well defined libraries implementing design patterns to solve issues they had before the refactoring efforts. They'll forever have my gratitude for that.

End results was an easy to maintain code and very extensible. But also it taught me a lot on how to architect things, what patterns to pick, etc. To this date, I've never worked on games with a code base as good as this one. Instead I'm doomed to see all the problems those games have in terms of architecture... (sometimes I can help solve some of them, when I'm granting enough time, but it's rarely a priority for companies, since there's no user facing changes and can induce regressions)

I had the chance to port the code to C# for some R&D in Unity, although I didn't really know C# at that time... but because the code base was so well split into libraries that made sense for the game, I could port them and test them separately and was able to progress much faster than expected. First with a client running as a console app, then later in Unity.

My love for that code base went as far as giving a lecture at the local University about its architecture and the patterns used in it :)

Fun fact: The libraries in Dofus are named after Discworld references, the world rendering library is named Atuin for example. A terrible idea in retrospective for new developers joining the team who had to idea what Discworld was!