31 is the highest IV a pokemon can get in a stat. You can talk with the IV judge in Kiloude City's Pokemon Center, if he says "Can't be beat" then the stats he list's all have 31 IV's.
Here's the list of natures
http://www.serebii.net/games/natures.shtml
You can see what stat they boost and which stat they lower. There's even some that don't give any boost at all. It comes downs to the Pokemon's stats take Scizor for example it can be a great Physical sweeper so it could have either Adamant(+Atk, -Sp.Atk) or Jolly(+Spd, -Sp.atk). Or a Defensive wall like Ferrothorn which could have Relaxed(+Def, -Spd).
If you don't want to get to far into the complicated part of competitive play of Natures and EV spread, go for a nature that'll boost the Pokemon's best stat and that'll lower a stat that doesn't effect it at all.