Ask HN: What is the best-designed software you've used?

Varnish

https://varnish-cache.org/trac/wiki/ArchitectNotes and https://varnish-cache.org/docs/6.0/phk/firstdesign.html

Don't forget Plan9

From the UX perspective of a long time user: GNU Emacs, I guess. AFAIK, its general architecture has not changed much in 30+ years. It was meant from the get-go to be extensible, a decision that has proven wise many times over.

Considering the obvious click-bait title, there are fundamental problems with the question imho.

How can you know how well some software is designed by using it? You can't. You need to see the code, to look at its specs, etc.

Of course, the OP might be taking about UX, rather than the other facets of software design, but even then there's a large disconnect from the design and the use.

As a user, all you can do is comment on metrics that are important to you: usability, speed/responsiveness, accuracy, stability, originality... At best, this might hint at the result of good design, but not always.

PS: My actual answer to all of this is: Software design can always be improved upon. There is nothing that I've written that I can't improve upon/do better after a re-design.

The Libby iOS app for access to public library content.

Also wins for best software funded by my tax $$.