Well lets see...
Zygarde-Well, no Lonely is not the best. If you REALLY want to add to that attack go for Adamant but ideally Impish would work well.
Mewtwo-Hardy doesn't really do anything. Better go with something to reduce SDef or Def because that's Mewtwo's worst stats. Mind you this is STAT wise and not really according to movesets.
Moltres-Yup, we would all say Rash probably work best stat wise and moveset wise. Modest WILL ALSO WORK though but then again Rash is recommended.
So yeah. For Natures click here.
Hope I Helped! =]