The top 5 most downloaded fl studio skins (as at August 2009):

1). Black Ninja Skin (6000+ downloads)
2). Strobed Skin Pack (5800+ downloads)
3). Energy Skin Pack (4300+ downloads)
4). The MF Shiny Skin (4000+ downloads)
5). jth noBack Skin (2200+ downloads)

This doesn't necessarily reflect the most popular skins, since some have been available for longer than others.

Note: A lot of my original statistics have been lost so there are more downloads than shown for some of the older skins.