Serif fonts, which have slight projections to finish off the stroke in each letter, are been proven to be easier on the eyes than san-serif fonts, which resemble block printing. Whether the font is proportional or monospaced also affects how easy it is to read. With monospaced fonts such as Courier, all characters are the same width. Proportional fonts make a manuscript look more like a book and allow more words per page but monospaced fonts give editors a more accurate feel for the space required by the piece. Size also matters, at least for fonts.
Pitch refers to the number of characters per inch.
Although any legible font might be acceptable, the safest choice is Courier 12. Work printed in Courier 12 closely resembles typewritten work. Familiarity with Courier allows editors to quickly extract word count and other important information from manuscripts printed in it. Photocopy the manuscript, back up the disk. Not vice versa.