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Don’t forget that size also matters. Studies have shown that larger fonts increase reading accuracy, but you don’t want your website to look like a child’s board book with huge letters. Try to find the happy medium, and avoid tiny font/size like the plague. Also, higher resolution screens make marginal fonts more legible, but you can’t assume everyone has a “retina” display.
One study I looked at (and there are plenty more if you do a little searching), indicated that the fonts with the highest subjective ratings were Arial 9.75 and MS San Serif 9.75. Both of these fonts also tested well for objective measurement of reading accuracy, with Arial performing slightly better. Don't take this as the definitive choice though - the study only compared a handful of different fonts.