Best preinstalled font: Malabar;
Best Custom Font: Bookerly, Linux Libertine G or Georgia Pro on epub;
Bitter on kepub because I have disabled kerning (and not enabled ligatures) to avoid uneven spacing problems that occasionally occurs on kepub and it gives very good result in these conditions.
Also like: IBM Plex Serif, Literata, Lora, Source Serif Pro on epub;
Sanchez Regular on kepub
Too much fonts installed?
I like to change the font depending on the mood: 32 custom fonts installed on Kobo and 27 on Kindle
added font: Merriweather, beautiful on both Epub and Kepub