The basic sweeper items would work. Things like Choice Band, Choice Scarf, Expert Belt, Plate, etc.
You could also use Weakness Policy, Leftovers, or Assault Vest if you wanted, as Mega Rayquaza has a good 105/100/100 bulk and Delta Stream, which weakens his weaknesses on his flying type side. Assault Vest and Leftovers are generally not recommended though.
Really, just use whatever item you want.
Good Luck in battle!