Alright from the testing I've done so far, this is what I expect viability to look like:
S Rank:
Reshiram - This thing is just about broken lol. It virtually has 0 counters, until of course you discover its actual set.. and by then it's too late. A sub set is intimidating as fuck especially when you take in consideration all of the moves it has access to. Substitute can use two different items pretty much, those being Leftovers and Life Orb / Flame Plate / STAB booster. A Life Orb set with sub / roost / stabs takes out a good chunk of the tier, as long as hazards are out of the way. Lefties would be used on a bulkier set such as sub / roost / toxic or wow / filler as a way to lure in special walls like Chansey. I haven't even touched on the likes of Flame Charge sets, or Choice sets. It can lure in special walls with Scarf / Band Outrages, or neutralize them with Specs Dracos. One more fun thing I see it being able to run is a support Tailwind set, which isn't even bad considering it's kind of like Flame Charge.
Yeah I don't see anything else being S rank right now, that is if you take the S rank description to heart. This rank should be reserved for the mons that dominate the tier. Does Reshiram provide multiple roles for a team or do one extremely well? Yes, in fact, it does all of its roles with quite a bit of success. Is the risk vs. reward more reward? Definitely, actually the only real downside I see to using this thing is if you truly want to build around a core and you see types stacking, or if you don't want hazard support for some reason.
A+ Rank:
(Some Arceus form(s) probably belong here but I haven't been able to test every single Pokemon in the tier yet)
Deoxys - Oh my goodness this thing is scary. I don't want to go into too much detail on its sets, mostly cause it only has 2, maybe 3 at most. First you have a solid wallbreaker set that can run LO or Mind Plate. Coverage basically comes down to what your team needs broken, but most likely you are going to have Psycho Boost on this set. Coverage moves include: Psycho Boost, Fire Punch, Hidden Power Fire, Knock Off, Zen Headbutt, Focus Blast, Shadow Ball, Extreme Speed, and many others. Next you have a lead type set with Stealth Rock, filler, filler, filler. Most likely you are running Focus Sash, and the 3 other moves come down to what you want accomplished by your lead. You could get Spikes up, Taunt to prevent rocks, Knock Off to cripple a threat, Screens, Psycho for damage, etc. Not to mention that Deoxys is the fastest Pokemon in the tier, even topping some scarfers. The final set is a Nasty Plot set, which is most likely going to be a nuke set if you want full power. This set could run a plethora of items including: LO, Expert Belt, Plates, etc. Coverage, again, comes down to what you need. The biggest thing I see is something like Nasty Plot / STAB / Focus Blast or Hidden Power Fire / Shadow Ball. No matter what set you use, you are getting a fast threat that hits hard, and with the lacking of checks and switch ins, this is worthy of A+.
Giratina - So I'll admit, I was way wrong when talking about Giratina before ORAS 2U was formed. I thought this would be the absolute best Pokemon, and ridiculously overpowered for the meta. After testing it though, I wouldn't say that it wasn't good because that would be a blatant lie, but it was extremely underwhelming compared to what I thought it was going to be. However, this is not to keep it from being A+ rank due to its sheer unpredictability, natural bulk and stellar typing, and its vast movepool. Giratina is like Reshiram in a lot of ways, basically being the exact same skeletal mons, just Reshiram is offensive, and Giratina is defensive, and each can run the other's set for decent success. I'd say the main reason why Giratina shouldn't be S is because of its lack of reliable recovery.
Kyurem White - Yet another extremely powerful dragon type Pokemon. Kyurem-White is probably the strongest Pokemon in the tier, being able to 3HKO (if I remember correctly) Chansey lol. It's basically a stronger Reshiram, with a different STAB and a wider movepool. The reason it's a rank below Reshiram basically comes down to Ice being such a hindering typing, and its inability to run a more defensive set. Other than those flaws however, Kyurem-White is definitely the strongest and one of the most threatening Pokemon in the tier, which is why I belive it deserves A+ rank.
That's what I personally see as top 4 mons in the tier after all of my testing. A rank will probably be giant, considering all of the great mons in this tier. I see a couple Arc forms, Azumarill, either Aegislash / Mega Metagross / Mega Scizor, and many other Pokemon. I really like the tier so far and I hope to see others get into it.
well if / when we pick up in interest in ORAS it should get a look at I feel