None. Reason: Every user will have a different opinion of which font is most legible/clean. Therefore, no one font selected by you will meet every users need.

None. Reason: Every user will have a different opinion of which font is most legible/clean. Therefore, no one font selected by you will meet every users need. Therefore, do not set any font, and allow the user's chosen preferred font, as configured into their browser, determine what font is used on your site.