Short answer, but I really disagree Latios being good as B- mons. Take a look at this calc:Ew, Vr
I'd argue that regular Latios is pretty nice right now, specifically this set:
Latios @ Waterium Z
Ability: Levitate
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Psychic
- Surf
- Recover
- Defog
It works really nice on bulky offences, working both in a defensive and offensive core.
Latios has a nice typing allowing it to switch in on current common mons such as Heatran, Toxapex, Gliscor, Tangrowth, Zapdos* and Rotom-W* (*can paralysis), as well as being a nice quick check to a lot of the tiers common choiced locked moves, in being able to switch in to Kokos Tbolt, Karts Swords, Greninja Water stab, Lando Eq, though it is to be noted that they can predict you in turn and ko you, however the options there. While not only providing for for the defensive core, it also works as a great defoger currently as it can come in on common hazard setters in Heatran, Gliscor and Toxapex for near free to remove hazards, while pressuring Rocks LandoT through revenging and can somewhat lure in Tyranitar Mega* (Surf into Hydro Vortex does 86.1min to Ttar Mega, so 98.6 after rocks, Surf into regular Ttar then Hydo Vortex on the Mega does 91.3min, Hydro Vortex in to regular Ttar then into Surf does 96min. So either Mega Ttar is picked off after Rocks damage, or is left severely weakened with no Rocks).
Latios also contributes nicely to the teams offence as well, luring both Heatran and Tyranitar for the teams game plan, while its speed and attack allow it to work also as a revenge killer for anything under its speed. Slower bulkier mons also have a hard time wearing Latios down too, allowing Latios to slowly dish out damage throughout the game, weakening answers such as Fini, Magearna and Spdef Bulu, applying pressure well throughout the match.
I'm not sure if the meta has shifted to make Latios good, or just that it has been forgotten about, however Toxapex, Heatran and Gliscors current high usage work only for Latios's benefit, so I'd argue Latios is as viable as most of the mons in B-, so Latios (Regular) to B-
252 SpA Latios Hydro Vortex (175 BP) vs. 224 HP / 252+ SpD Assault Vest Tyranitar in Sand: 124-146 (31.2 - 36.7%) -- 66.4% chance to 3HKO
This does pitiful damage, and Latios is honestly a horrible lure for one of its biggest checks. Not to mention both this and its Mega forme are at a new low, since Magearna and Mega Alakazam is common as hell and they practically wall this set.
As for comparing this to B- rank, that's just unbelievable. Amoonguss is actually really good in the meta rn, Zard Y is a fearsome wallbreaker, Rachi is a decent scarfer/spdef wall esp for psychic spam, Keldeo has a decent niche in breaking past Pex with CM + Taunt, Manaphy is a niche screens abuser and rain abuser, Pyukumuku and Quagsire are the only Unaware users aside from Clef, and Slowbro and its Mega forme are amazing physical walls, stopping Pokemon like Mega Medicham and Mega Lopunny while maining offensive presence. Latios is horrible compared to all these Pokemon, and the set you mentioned is just quite unviable.
You honestly do more damage with uninvested Z-Quake:
0 Atk Latios Tectonic Rage (180 BP) vs. 224 HP / 0 Def Tyranitar: 218-258 (54.9 - 64.9%) -- guaranteed 2HKO
I say the Eon twins should drop to C, having practically no usage in tours and generally outclassed in any roles. Offensive Scarf Latias is outclassed by Jirachi as a healing wish user, and Scarf Defog is horrible, ruining your momentum. Offensive CM Latios is walled by a bunch of stuff like Celesteela and Magearna, and is generally a waste of a Z-Move slot and why are you using Defog on Latios when you have a bunch of new Defoggers like TornT, LandoT, or Gliscor?
E: ok dang this wasn't short at all