vespa and or derbi (variated counterclutch bikes) i'd say tie for most potential ever. that has nothing to do with part availability in the states however.
then peugeot (variated swinging setup)
followed by motobecane ( exactly like peugeot but less reliable) then puch (most aftermarket support of all with a simple clean design ripe for modding) almost a tie tho IMO
minnerelli (check out what they are doing ins South America with these)
the lower ones are all around the same however, honestly any bike has the potential to be a champ some are just easier.
if the list was purely parts based it would look more like this:
motobecane av10 (euro import only)
motobecane av7 (usa motobecanes)