This is a two part question, the second question is dependent on the first:
What are the most popular fonts used on web pages? (the top half dozen will suffice).
Include the proper CSS fallbacks for the above fonts.
The reason I ask is that I have to build a form that will allow webmasters to choose a CSS font from a drop-down menu. The site will take the chosen string (fonts with respective fallbacks) from a database and insert it (via php) into aspects of a page.
Edit: Just passing this on. Although not scientific in its approach to what constitutes 'popular' fonts and respective fallback combinations, this was a helpful starting point: http://www.w3schools.com/cssref/css_websafe_fonts.asp