I once read an ebook that was formatted by a guy who had only ever done magazines and it was a huge mess. 2 columns of text per page. No auto scaling.
The best ebook format I have ever experienced is .txt and just let the software figure out where the text needs to go.
Many ePub readers allow you to ignore formatting. And that’s exactly what I do.
The best format was fb2
Why "was"? I still read books in fb2.