It seems to me Primecoin is the most differentiated coin.
this is true, but is that
better?
it depends what you want in a coin... for example feathercoin has a hard limit on the number of coins that will ever exist, while primecoin does not
if i had to take a guess, though, i'd guess that in the long run, primecoin will have a higher USD valuation than feathercoin (is that your definition of 'better'?)