If, like me, you have astigmatism, you may find a sans-serif typeface (such as the built-in Avenir Next or Gill Sans) easier to read than a serif one. Serif ones don't bother me on printed books, but they are harder for me to read on E-Ink displays or computer screens.
My current favorite is Noto Sans, which is free from Google:
https://www.google.com/get/noto/
From the Noto Sans zip file, you want these four files:
NotoSans-Regular.ttf
NotoSans-Italic.ttf
NotoSans-Bold.ttf
NotoSans-BoldItalic.ttf
Put them in a folder named "fonts" -- all lower case below the root directory on your Kobo, and give them a try.
Best of luck!
Rob