I think many designers in general are prone to using fonts and sizes that are fashionable and suited only to those with very good eye sight, as in the young. If you work with older users and I'm sure users with less than perfect sight, or poor sight. You realise theres an awful lot of people that just can't read smaller fonts. Or off beat colour combinations. Most of these designs are never tested against users properly.
I find the best compromise is Arial. People are used to it, so it doesn't look out of place on formal letters, and obviously its very clear on a monitor. If you were trying to create a specific impression, legal, financial, modern, you might use something else. It very much depends on the contex.
So the answer to best font has to be "it depends"