Rash increases Special Attack and lowers Special Defense.
Milotic's base stats are:
HP: 95
Attack: 60
Defense: 79
Special Attack: 100
Special Defense: 125
Spee: 81
As you can see, Milotic's best stat is it's Special Defense. Therefore having a nature which reduces this is not the best nature. The best possible nature for Milotic would be either Modest (+Special Attack, - Attack) or Calm (+ Special Defense, - Attack)