Whatever a book's default font face is (labelled as
Publisher Default in the dropdown list in Kobo's settings), I won't change it.
You know why? Because if you change it to Kobo Ember or Calibri or whatever, then you lose the variety of fonts that come with "Publisher Default".
For instance, I was reading a book last night whose chapter headings were sans-serif and block-like and whose main text was svelte and serif. If I go with something other than "Publisher Default", I get a boring, homogeneous look. Yes, the content is priority, but I now appreciate appearance/presentation more than I did years ago.
Who's with me?
UPDATE: I've uploaded a picture of the "blocklike chapter headings" and the svelte and serif body text.