Since its attack and special attack both are 438 when maxed out with a beneficial nature I'd say either Adamant or Modest. But if you want speed over attack you should get maybe Jolly, personally I like the Modest nature and I know many people also go with that.
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Rayquaza
Rayquaza and deoxys are the best ... or at least for me....
try master ball it is the best olviously
You get RAYQUAZA at the top of SKY PILLAR after seeing GROUDON, KYORGE and RAYQUAZA in SOOTOPILIS CITY. RAYQUAZA is LV.70 so be prepared! RAYQUAZA knows these techniques: EXTREMESPEED REST FLY DRAGON CLAW. The ratio of it being shiny is 25%. The only difference between SHINY & NON-SHINY is that the SHINY one is BLACK, not the regular GREEN. The best ball to use on RAYQUAZA is either an ULTRA-BALL (Or if you are lazy and not bothered a MASTER-BALL!)
RAYQUAZA IS THE BEST or GROUDON or KYOGRE for Pokemon Emerald THE 3 LEGENDARY!