I am currently alternating between the Kobo Nickel (with the weight turned up a few notches in Advanced settings and Amaranth (a downloaded font, see below). Amaranth is a naturally "weighty" font and it also comes with an Italic version as well. The omission of Italic versions of Kobo fonts is frustrating and greatly lessens the reading experience. Instead of italics it displays the font in bold. Bold in written text I see as the equivalent of "shouting" which is most distracting. If the author put text in Italics, this is how they should be displayed.
The Amaranth font 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). The only disadvantage of a downloaded font is that Advanced options, for advanced tweaking, are not available from the Kobo menu.