Natures
Hardy does not change any stat. A Pokémon with this nature has exactly the values that follow from its base stats, IVs and EVs.
A neutral nature — it changes nothing. If you are looking for the best neutral one, all five work the same; only the name differs.
Flavor
None — a neutral nature has no liked or disliked flavor. That is convenient with Poffins and Pokéblocks: such a Pokémon eats anything without complaint, so healing never works worse.