completed 5jun2013 - liked the "Lato" sans serif. "PT Serif" is okay, but I still prefer sans serif. Medium.com's serif is the best serif that I've seen. I'll stick with Open Sans, but I may test Lato some more. Need to increase the font size with Lato and PT Serif.
http://mattgemmell.com/ -- uses Google fonts PT Serif and PT Sans.
http://summerofdesign.com/ -- uses Google font Lato.
Example code from summerofdesign.com blog site.
margin: 2.8rem auto 0 auto;
font-family: "Lato", "Lucida Grande", "Tahoma", sans-serif;
color: # 333222;
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