Short answer: Mega Rayquaza is in the topmost echelon of Pokemon when it comes to speed w/o being greater than FTL.
Long answer: At the bottom is
Poliwrath, the one with the feat, who isn't even the fastest fully evolved Pokemon, as Pokemon like Jolteon, Manectric, Lucario, and Weavile exist. Any fully evolved Pokemon would get lolblitzed by a Mega Pokemon. And a Mega Pokemon would in turn get the same treatment by any Legendary, take Raikou, Zapdos, or Lati@s. Then there's Mega Lati@s, who'd lolblitz its base form (by the same logic as above), who are in turn, outsped (not blitzed in the slightest though) by the Primals. Rayquaza in base blitzes the Primals, and is widely considered to be one of the fastest Legendaries period. Mega Rayquaza lolblitzes his base form via Mega logic.