I have tried a few and my current favourite is the Amaranth font which you can download from here
http://www.fontsquirrel.com/fonts/li...e/Sans%20Serif . Create a folder called fonts in the root directory of your Kobo and then just copy the downloaded fonts into this directory, the Kobo will then have Amaranth as a font option (explained better here
http://www.youtube.com/watch?v=KU7JpaKlyMc)