DeviantArt does not offer an option to browse skins by downloads so here is a list of the top 20 most downloaded Winamp Classic skins with publication years and download numbers up to today.
Tiger (color pack, 2004) 82,610Dark Materia (2005) 71,155metrix metal dream (2001) 42,670Milk 2 (color pack, 2004) 34,925Aluminum Alloy (color pack, 2003) 28,917Longhorn Slate (2004) 28,426Energy Amplifier (2002) 23,734Aesthetic Groove (color pack, 2005) 23,131Elemental (color pack, 2006) 22,254gfxOasis (2004) 21,061Longhorn Inspirat (2004) 21,054Azure (2004) 20,780Zen (2007) 20,026Nucleo Nlog v2G (2001) 19,507Inspirat SE (2005) 19,452Kuantum (2005) 18,0