There really is no "best nature". All the natures seem exactly the same to me. If the pokemon is a feebas and you want to evolve it into a milotic, then that nature that makes it really like dry pokeblocks/poffins is good, but if you want to evolve it the hard way by getting a different nature, then that's fine, too. And also, why do you think the pokemon company would make bad natures?
~!~Miss Cow~!~