Most definitely Scolipede and Mawile imo. Scolipede was already a pretty solid Pokemon in the lower tiers, but x/y Scolipede seems terrifying to fact to say the least. Speed Boost is an amazing ability, and Scolipede utilizes it pretty well to outpace scarfmons if trying to sweep or attempting to pass some Speed boosts along with maybe a Swords Dance like fellow Speed Boost users Blaziken and Ninjask, and its almost definitely better than the latter as a whole. Not to mention, Scolipede now has base 100 Attack instead of base 90, so its Megahorns and such now pack a bit more oomph.
Mawile was regarded as a pretty mediocre Pokemon until x/y, and x/y gave it some pretty massive buffs. It gained a whole new interesting type in fairy, that happens to pair well with Steel too, along with a mega form with buffed defenses and the ability huge power, which ensures it can pose a threat to most teams. Mawile can even Intimidate before it mega evolves the first time iirc, which is of course great since it makes her much easier to switch in.