Gamasutra notes there are roughly 30 PS1 downloadable classics on the US PlayStation Store -- a number Sony promises to increase. From looking at the chart embedded above, we can see which titles are the most popular.
Crowning the top ten is Capcom's Street Fighter Alpha, which debuted back in August of last year. It's also not surprising to see Konami's Castlevania: Symphony of the Night near the top; however, it is interesting to note that two third-party titles are easily dominating the top spots, despite the fact that most available downloadable PS1 titles are published by Sony (including the eight other top ten games). If only third parties would react to this data, we'd have a more diverse library of classics.