I could say "None of them" and suggest that we get the 'designer' world involved in making an even better one.
But the interface for doing that is currently very poor, in a number of ways, as is pointed out on http://wiki.audacityteam.org/wiki/Theming_HowTo
. + editing the PNG and then reloading into Audacity involves a number of steps so slows down any 'what if' attempts - an 'Apply' button in the prefs would help.
Incidentally the <-> icon in the Tools Toolbar is wrong as it can be used to move clips up and down as well ('obviously' if there is room). It has been wrong for many years and this could be an opportunity to fix that.