Kyogre is at the top of Uber usage for good reason. Politoed will be number 1 OU mark my words.
Yeah, 150 BP + STAB + Rain + Specs + 150 Base SpA = Uber Kyogre.
Politoed has Drizzle alone.
Anyways, I don't think there will be a single most impactful pokemon, rather, particular groups.
With the newly revealed dragons and the recently updated powerhouse dragons, I'm definitely seeing a Dragon/Steel metagame being reborn.
-Ononokusu the CBer (face it, this thing is too frail to be a setup sweeper).
-Sazando the Specs abuser (no Pursuit weakness, decent bulk).
-Latias/Latios the CM twins (Sazando does the Specs better, IMO).
-Garchomp the SD sweeper (bulk + speed = win).
-Flygon the Scarfer (Immunity to all hazards is niiiiiiice.)
-Salamence the LO Mixed Attacker (It outmuscles all other dragons).
-Dragonite the bulky booster (Light Screen, Roost, DD, new ability).
-Kurimugan the defensive dragon (77/90/90 mono dragon is awesome).
-Altaria the support dragon (Heal Bell, Natural Cure, Cloud nine).
Clearly, we have a fleet of dragons available to fit a wide variety of roles, which will make steel typing all the more important. Aside from the typing Metagross/Jirachi/Skarmory, I'm sure we'll be seeing that Fighting/Steel dog, who's bulk and power allow it to defend and threaten.