Adamant. Zeraora's speed is already over 300 with 0 speed EVs and an Adamant nature, give it 252 speed and 252 attack, and it's attack would be 355, and it's speed would be 380, aren't those decent stats compared to a Jolly Zeraora, now a Jolly Zeraora has 423 speed (that's pretty high), but for the high speed, it costs it's attack to drop to 323.
I find a lot of times that 300 speed really isn't enough to out-speed most important/fairly common fast things - Stakatakcool
I don't get it, If 300 speed isn't enough to out-speed Pokémon, then 300 attack is just the same, right? If you have high speed, sure you can out-speed Pokémon, but you only have 323 attack, what are you gonna do if you find an Aggron, Slowbro, or Cloyster? They all have higher defenses than your attack. I'm pretty sure Adamant is the best Nature for Zeraora unless you use Bulk Up like mine, then I suggest a Impish/Careful Nature.