I've done this for years and swear by it.

Top 5 codebases for changing my mind about things:

Wietse Venema's Postfix mail server. Taught me tons about security posture, the architecture i'd describe as microservices before microservices was a thing, but contrary to the modern take on microservices (it's mostly a tool for decomposing work across large semi-isolated groups) this was primarily about security and simplicity.

Spring framework - this opened my eyes to ways of working that i hadn't really thought enough about before, the developers on that project have a culture of deeply considering the needs of their users (who are java developers often in an enterprise environment).

Git - the thing i like about the git code base is that once you've covered the objects database (e.g. blobs, trees and commits) and the implementation of refs, everything else just feels like additional incremental features. With those core concepts, everything else is kinda harmoniously built on top.

Varnish by Poul Henning-Kamp is another one - feels like he went to great lengths to make that code base a teaching tool despite the fact it's also a top tier reverse proxy.

Last one isn't a code base - but it will help with software design in the large; studying how the lieutenants model works in the linux kernel.

Thinking about my answers, i think i've highlighted something subtly different than "well designed codebases" it's more a list of codebases that left a notable long lasting impression on me because of design decisions they made.