Greninja is way to frail to even be S Rank. Its bulk is non existent and it faces with a plethora of 'mons depending on its last slot. You have even things that can set up on it like Gyarados, Clefable, Mega Tyranitar to an extent..., and priority and Choice Scarf is omnipresent. Walled by Alommomola, Azumarill, Rotom-W, and most bulky water-types is a pain as well. The Spikes set is good, but it is giving up a valuable slot. Its definitely one of the best PKMN in A+ Rank, but those flaws are not getting it to S Rank. It's facing these challenges and as the metagame progresses, we will see more of these users. Its a huge threat, but it's not deserving that rank. Keldeo is definitely worthy of S Rank. It can be easily slapped on any team to be a wallbreaker, revenge killer, CM booster, etc. It fills up many roles as well. Probably the worst 'mon in S Rank, but still a worthy candidate of it.
Tornadus-T needs to be bumped up to A- Rank ASAP in my opinion. I've nominated this thing to A- a million times this generation to be ignored by everyone. The bird has been a powerful tool for both stall and offensive teams which allows it to be used in unbreakable Regenerator cores or SPAMming powerful Hurricanes. I'm going to break this analysis into separate points.
1. Tornadus-T fits onto
many teams with very
little opportunity cost
This is the biggest draw out of using any PKMN. This things ability to fit on BirdSPAM, VoltTurn, Rain Offense, Stall, and [Semi] Bulky Offense easily makes it an excellent choice on any teams. Having low opportunity cost is great too. Regenerator allows it to continuously use U-turn, Knock Off / Taunt, and Hurricane through out the game is great making it a wise choice for a pivot. Regenerator either (or both) [A] allows it to avoid Stealth Rock gets it out of Life Orb trouble [C] recovers health from switching into moves like Keldeo's Secret Sword or tanking Bisharp's Sucker Punch [D] gives a useful pivot for offensive teams trying not to lose momentum, power, or something that can't tank hits all the time like Rotom-W. Combine all these facts with the Aegislash Ban, ability to use a powerful Hurricane, and make great cores, Tornadus-T is already standing high in its position.
2. Useful Movepool
Tornadus-T's movepool makes it a unique threat for the opponent. Its speed tier is great and it can abuse moves like Taunt, Knock Off, U-turn, and Rain Dance really well. Hurricane may have 70% accuracy, but it still is a good tool to threaten the offense. The main argument I am seeing is that Hurricane is poor so it is not worthy of A- Rank.
Valmanway If you are basing your argument at this alone, you clearly have never used it. How about we drop Keldeo because it most powerful STAB only has 80% accuracy? Nah. Hurricane remains a powerful move that can put your opponent into a checkmate position due to it lack of resists and if your opponent does have one, Knock Off or Superpower is going to be crippling / beating those 'mons like Chansey, Tyranitar, and Thundurus-I (on switch).
3. Tornadus-T fairs well against
most of the metagame
This is the ultimate reason why you use this thing. Its coming in on things like Mega Venusaur and Chansey to make them crap their pants and its speed tier allows it to revenge kill dangerous threats like Garchomp and Latios (assuming they are weakened a little bit). U-turn forces in counters / check to make your opponent lose their momentum. A fast U-turn especially paired with Knock Off / Taunt. It has excellent coverage move to beat like Heatran and Mega Scizor. Assault Vest sets are amazing pivots that can tank repeated hits, etc. If we never had a metagame like this, tornadus-T wouldn't be going past B Rank this generation.
There are other points, but these are the standard ones. The increasing usage of this thing is the fact that people are realizing these things. We have things like Mew in A-, but not this bird?