Chiswick Marketing 45,000 readers took part in this experiment. And the answer is use Baskerville as your font of choice if you want to be understood more.
This is interesting as it is an old font and originally designed for print, but the experiment was the on-line edition of the New York Times.
There is another experiment mentioned in Malcolm Gladwell’s latest book, David & Goliath, that shows that when instructions are more difficult to read then they are understood more. That theory says if I have invested time in reading and understanding this then I will follow the instructions better. So maybe Baskerville works as it is not a clear as other web fonts and so we invest more time in reading it and so understand more.