I remember also a thread about preferred fonts in Kobo (or in general not specific for Kobo?), but I will answer here.
I use mainly Bookerly for epub, Merrywether and Bitter for Kepub. You can find Merrywether and Bitter in Google fonts but the second has been updated on its site and last time I checked was not still updated in Google fonts. For Epub I also love Literata. I use other fonts too, I like to change them and make experiments.
You can find plenty of freely usable fonts for example in Google fonts:
https://fonts.google.com/
In the SIL fonts project:
https://software.sil.org/fonts/
In Tex Gire project fonts:
http://www.gust.org.pl/projects/e-foundry/tex-gyre
In the Latex archive fonts (not all available in the ttf or otf format):
https://tug.org/FontCatalogue/