Most popular card won't mean most practical to benchmark, so your "on the market" line is of most importance.
I haven't seen any sales figures that would rank current cards by popularity. The Steam Hardware Survey does help see a trend of what's being used. Click on Video Card description to see the complete list. GF 8800, GF8600, HD 4800 series lead the pack. http://store.steampowered.com/hwsurvey/
Thing to keep in mind with Steam though is their games in general do not require top notch components and loads of gamers still play Valve games regularly on aging hardware. It sure wouldn't match the list of GPU's used for Crysis.