Azumarill ----> S Rank
This thing provides both incredible offensive and defensive utility for your team at once its unreal. With people spamming Rocky Helmet Garchomp, Landorus-T, and Hippowdon as their main go to physical tanks, many teams struggle to find a decent switch in to Azumarill. In fact, Azumarill can come in on a myraid of Pokemon such as Garchomp, Latias, Weavile, Heatran, Keldeo, Tyranitar, etc. and click a move and just watch something on the opposing team die. With the severe decline of Mega Venusaur (who is incredibly easy to wear down with chip damage anyways) and gradual wane of Mega Scizor due to the prevalence of Tank Chomp/Mega Manectric/Talonflame teams, Azumarill finds itself putting in tremendous work mostly every single game. The CB set (the best one currently imo) is a nightmare for offense teams since they'll most likely be saccing a mon every time Azumarill comes in, and they can't go to Tank Chomp like they do with most other physical threats. Besides Garchomp, there are a myraid of other mons on offense Azumarill can come in on, including Latios, Keld, Bisharp, Weavile, Dragonite, Excadrill, etc. Vs stall teams, the CB set still does major work as it threatens out Mega Sab, and 2HKOes the relevant walls in Slowbro/Skarm/Quagsire with the appropriate move. Rotom-W, one of the mons people just slap on their team to "wall" Azumarill just gets 2HKOed by Play Rough/Superpower, and Ferrothorn is just a 50/50 depending on if you switch into Superpower or not. Belly Drum is one of the scariest threats for stall to face that don't have an unaware mon and can win games in an instant. I also don't know why everyone thinks Assault Vest is a shit set not worthy of running atm when it's actually pretty underrated and destroys offense. I get that its still really scary switching into Keldeo because of Scald burn chance, but once you look past that, AV Azumarill can pretty much take on every member on offense 1 v 1 such as Tornadus-T, Thundurus, Gengar, Alakazam, Gardevoir, and weakened Serperiors. Azumarill is without a doubt one of the hardest mons in the metagame to switch into.
Now obviously hitting extremely hard isn't enough to warrant a raise to S rank, but Azumarill boasts incredible defensive utility as well. For one, it provides you with extra assurance against both Rain and Sand teams. The ability to resist Kingdra's STABs, KO back, and use the rain boost to hit everything on the opposing team even harder and a buffed up priority to take on Kabutops & Tornadus-T is priceless if you're running any form of offense which usually has trouble with rain. The ability to immediately threaten Tyranitar/Hippo and stop Sand Excadrill from sweeping you as well as being a back up Keldeo switch in is also invaluable for any form of offense. Besides all the aforementioned mons, Azumarill can act as an emergency Zard-X check (we all know how hard those are to come by these days), Lopunny check, Weavile check, and can also take on Clefable by outspeeding and 2HKOing. There is practically no risk to using Azumarill because it will always put in work every game since the only two counters it has (Mega Venusaur & bulky Mega Scizor) are declining in popularity and nothing else in the metagame can sufficiently switch in safely. That to me is the definition of an S ranked threat.
Keldeo ------> S Rank
Similar to Azumarill, I believe this thing also fits the criteria for being S rank for the exact same combination of reasons (simple & risk free to use, provides both offensive power and defensive utility, puts in work every single game). However, while Azumarill has pretty much no guaranteed switch ins, Keldeo actually has plenty of common switch ins roaming the tier, yet it still has the capability to muscle pass all of them regardless.
The amount of Pokemon Keldeo can come in on and fire off a free attack against is ridiculous. Heatran, Hippo, Gliscor, Ferro, Skarm, Chansey, Landorus-T, Bisharp, Mega Scizor, etc: are all immediately threatened out by Keldeo and give it a free turn to attack or set up. Now, combine that with how Keldeo can cripple its common switch ins with a lucky Scald burn and how easy it is to chip away and wear down these mons, and its easy to see how Keldeo has always been a top tier threat. Keldeo can pretty much OHKO/2HKO the entire metagame unless your opponent has a slowbro/latias/venusaur/amoongus/starmie/torn-t/latios/altaria etc. Latis & Starmie can easily be trapped, rendering your only Keld switch in useless, Vensuaur is easy to wear down with burn damage, hazards, sandstorm, and shitty 8 synthesis PP, and Slowbro & Mega Altaria can actually get 2hkoed by specs hydro pump on the switch once its burned, factoring in SR. The ease in which you can get Keldeo into the game and severely weaken/cripple a Pokemon right from the start is why many people usually lead with Keldeo.
Besides being an offensive powerhouse, Keldeo offers a good typing defensively and can act as an extra defensive backbone against many common threats. Its one of the most splashable Bisharp & Mega Scizor counters in the tier; one of the few Weavile checks on offense, checks Crawdaunt, Mega Tyranitar, and Mega Gyarados once it has evolved. It can also act as a desperate offensive pivot to Kyurem-b/Hoopa-U if you predict them firing their main offensive STAB and threaten to OHKO both in return. Also like Azumarill, its added insurance vs rain teams, since you can check Omastar/Kabutops/Mega Swampert and nothing on a rain team wants to switch into a Keldeo for obvious reasons. Keldeo's ability to come in numerous times throughout the match easily and slowly pick apart its switch ins, while OHKO/2HKOing the vast majority of the metagame is what makes it a S ranked Pokemon in my eyes.
Azumarill and Keldeo are both top tier threats in the metagame currently and everyone must be prepared for them. They practically define the current state of ORAS OU and have consistent high usage due to their splashability, offensive stats, and defensive utility. While Manaphy is perhaps the scariest out of the three for defensive builds to handle due to its abundance of sets and type coverage, Keldeo and Azumarill are overall much more consistent vs a variety of styles and will always put in work regardless. There is no reason to have Manaphy one rank higher than them. Raise Azumarill and Keldeo to S rank.