Chris D'Arcy Bean • Hi Ed - if you are embedding fonts, then any open-type font should work. You will need to test them with different display types, magnification methods and possible printed output etc...
If you are trying to get the best all rounder for free, I think you need to stick to "Arial, Verdana, Droid Sans" for paragraph text chunks. You can use "Times New Roman, Georgia, Droid Serif" for headings that are a larger font size.
for a reference of common fonts on OS variants, look here:
Windows PC from XP
http://www.microsoft.com/typography/fonts/product.aspx?PID=135
Apple iPad/iPhone/iPod
http://www.michaelcritz.com/2010/04/02/fonts-for-ipad-iphone/
Android
http://www.droidfonts.com/droidfonts/