Landorus-T to S Rank on Flying
I think that Lando-t does so many roles so well on flying that it can't be ignored and left in A rank. Its defensive typing allows for neutrality to rock and an immunity to electric helping a lot vs Victini's bolt strikes and Terrakion's Stone Edges. Intimidate and access to u-turn makes it the perfect pivot. Lando usually is scarfed or defensive, but a double dance set is also somewhat popular. Intimidate helps wall mons like terrakion, even on the less bulky sets.
Landorus-Therian @ Choice Scarf
Ability: Intimidate
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- U-turn
- Rock Slide/Stone Edge
- Earthquake
- Knock Off/Superpower
-1 252 Atk Terrakion Stone Edge vs. 0 HP / 0 Def Landorus-T: 118-141 (36.9 - 44.2%) -- guaranteed 3HKO
Also, this:
-1 252 Atk Victini V-create vs. 0 HP / 0 Def Landorus-T: 178-211 (55.7 - 66.1%) -- guaranteed 2HKO
Scarf landorus still lives a -1 V-Create, and either outspeeds and ohkos next turn or forces a switch.
Landorus-Therian @ Rocky Helmet/Leftovers
Ability: Intimidate
EVs: 252 HP / 252 Def / 8 SpD
Impish Nature
- Earthquake
- U-turn
- Stone Edge/Rock Slide/Knock Off
- Stealth Rock
Running rocks on a bulky lando allows skarmory to run spikes instead of stealth rocks, and it's neutral to fire, the only skarm weakness that isn't covered by the electric immunity. Skarmory in turn provides a neutrality to water and ice, landorus's only weaknesses.
Landorus-Therian (M) @ Leftovers
Ability: Intimidate
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Earthquake
- Stone Edge
- Rock Polish
- Swords Dance
+2 Speed with adamant allows it to outspeed scarf base 120swhich you'll never seeand dishes out more damage. This set helps tremendously against rock, which gives flying a hard time otherwise. Cradily is basically setup bait because it hardly touches lando, and after both dances it can solo rock.
+2 252+ Atk Landorus-T Earthquake vs. 252 HP / 0 Def Cradily: 399-469 (106.1 - 124.7%) -- guaranteed OHKO
Double Dance Landorus-T can also set up a Rock Polish on Mega Diancie, and sweep from there.
Refer to the scarf Victini calc, it can also set up on it after Lando Intimidates it, then can solo fire. It can also obviously set up on a Kyurem-Black locked into Fusion Bolt, either SD or RP depending on what you think they'll switch into, and sweep from there. This set helps particularly well with these two types.
In conclusion, I think that the options Landorus-Therian offers to flying are too much to pass up and it should be moved to S rank.
Part of the issue with Landorus-T is that you can't run Landorus-I.
There isn't much the Choice Scarf set particularly does for Flying. It fails to outspeed the key Choice Scarf users Victini and Kyurem-B (Like all other notable Choice Scarf users on Flying). And it's not like Flying doesn't have many good physical attackers, with Dragonite and of coures Mega Gyarados being very popular to combat Psychic. Without Mega Charizard Y, Flying doesn't have any strong special attackers (yes Nasty Plot Togekiss and Thundurus exist, but both need to set up to be powerful and both also give up using other sets that might benefit the team more), which is why Landorus-I is so important. Certainly on a Mega Charizard Y team, it'd be easier to justify using Landorus-T, but given the current usage of Mega Gyarados, I find Landorus-I is simply superior for its special wallbreaking.
Personally my favorite Landorus-T set is the defensive rocks user, although I use Leftovers for recovery. Even then, the one niche I find this set to have is to allow the Skarmdos core to both run Defog, and to more reliably put up rocks vs Fire. That being said, Landorus-I can also run Stealth Rock, and because it forces many switches with how threatening it is, it will have surprisingly many opportunities to use it (Victini's V-create cannot OHKO Landorus-I, on a side note for the Fire matchup). I don't find this to be a very compelling reason to use it. Having the extra physical wall is nice in some circumstances in corner cases, but isn't enough to justify its usage over Landorus-I on Mega Gyarados teams.
The final set is the Double Dance set. I'm not that sure this set is even useful, especially with your given examples. It seems like a "win more" set in the few cases it even works. Against Psychic teams with Victini (one of your examples), Slowbro walls this out completely, so it's not even a real threat. For the Kyurem-B example, it should be noted that any good Kyurem-B user will be clicking Ice Beam instead of Fusion Bolt almost every single time, so the scenario presented is rather rare. Nevertheless, on Ice teams, Avalugg just comes in and walls Landorus-T forever. And even against Dragon teams, unless Landorus-T somehow gets both a SD and a RP, Kyurem-B can come back in to live a Stone Edge/outspeed Landorus-T and OHKO with Ice Beam. Against Rock, Iron Head Skarmory sort of already just wins anyway? And even if something went wrong, you still have Mega Gyarados too which just wins too. So this is the prime example of a "win more" case.
For me personally, at least, I have to think of a reason to not use Landorus-I in order to use Landorus-T. I just don't see it being S rank as a result; maybe in a metagame where Mega Charizard Y is more desirable than Mega Gyarados, it could be.