Seen HTML5 Boilerplate talk by Paul Irish? verdana, Myriad and Arial, none of these are available on all three platforms. Best is to let browser decide which font looks best. This is the code they use:
/* Font normalization inspired by YUI Library's fonts.css: developer.yahoo.com/yui/ */
@shazdeh the reason why sans-serif is used in the boilerplate is because Arial looks better than Helvetica in Windows, and Helvetica looks better than Arial on a Mac. By using sans-serif, it defaults Windows to Arial and OSx to Helvetica. In the example I gave, sans-serif is a fallback in case Myriad pro and HelveticaNeue isn’t on the system.
@raman2572 I think you misunderstood what @jamy_za was saying. He has fallback fonts for computers that don’t have myriad-pro-1. This way most computers see his design as he planned it and those that don’t still get a sans-serif font presented.
Also, I would be careful saying 13px is the best stated by all. It might be recommended to minimize cross-browser rendering differences but it is not always the best font-size. As @snillor said, declaring a font size in pixels overrides browser font size options and some people might choose to have the font size increased due to poor eye-site or just increased readability.