Most readable for printer are Serif Fonts. So now I can’t choose from these GNU fonts: Libeartion Serif; DejaVu Serif; Droid Serif. Maybe you know some better fonts?
In my opinion fonts without serifs are better to read. Personally, I consider Helvetica most attractive. From your list, Liberation Serif would be my GNU font of choice. You can also look for TeX fonts that are available as .ttf-file.
@lactea maybe you meant that sans fonts are better to read from monitors? Well, I’ve just readden that Sans fonts are better for monitors, and Serifs for paper (Serifs help you not to distract your vision to other line). And Mono fonts for terminals, notepads 
I would use Linux Libertine G (under GNU/Linux) or Linux Libertine O (under other O/S). Both of these serif fonts are distributed with LO. Gentium Book Basic is another option (also distributed with LO) however I would advocate downloading Gentium Plus as it is a more recent version.