I have been reading a lot of discussion on Corviknight and as I already made a post about Corviknight being risen to A- so I won't nominate it again for that to avoid spam but I want to give my 2 cents on some of the things people are saying-
I don't think that corviknight is good as it was in dlc1, there's better defoggers out there (hydreigon, moltres, zapdos, tornadus-therian, lando-therian ) There's too many things in the tier that threaten corviknight (cinderace, moltres, zapdos, heatran, magearna, nidoking, slowking-galar, tapu koko, volcarona, urshifu, dracozolt, spectrier, tpunch melmetal, latios, np hydra, dragapult, barraskewda, specs lele, tapu fini) it cant defog on many things as there's so many threats that can easily beat it.
Corviknight is still a very good defogger because it doesn't need to defog on the list of mons that you gave. Corviknight is capable of directly switching in and defogging on the most prominent rockers in OU themselves. Lando T, Clef, Ferro, Garchomp, Tyranitar on sand and Swampert are the among the most common rockers you will see. Corviknight completely walls all non setup variants of these and wins the hazard war with pressure. Chomp and Lando T often use SD and the latter sometimes even uses Smack down (This set is quite rare though) but +2 edge only does around 60% which corviknight can roost stall with pressure so they need multiple SDs against it in which time it can Uturn out to the appropriate revenge killer. Corviknight should win this in the long term. . SD rocks chomp will always have Edge-quake because it is good against all defoggers save Corviknight. Fire Fang is very rare and simply can't be afforded on rocker sets. Non boosted fire attacks from chomp bounce right off. This is a very crucial advantage Corviknight has over the other birds. All the other birds are very afraid of edge. They can't defog on these SDing behemoths. Another advantage Corviknight has over these birds is that it doesn't require boots and is not afraid of knock off and it is toxic immune. This makes it so that even defensive rockers like Ferro, Clef can just knock off others on the switch and cripple them heavily. A simple Spdef Pert can just toxic them on the defog and put rocks right back up. Ttar doesn't need to SD to threaten bird with it's rock move. These birds often don't run pressure either so it is a losing battle in the long run.
One common rocker that Corviknight doesn't beat is Heatran. But to be fair only hydreigon and Moltres beat it out of all the mons you mentioned.
The problem with hydreigon is that it is exclusively ran to counter all heatran and spectrier. The normal spdef set with dark pulse, earth power can not dish out too much damage itself and loses to the other rockers like Clef, Leech Seed Ferro, spdef Pert, etc.
Lando T doesn't have any recovery and is used as a rocker more often than a defogger. Defog is usually seen on scarf sets as an emergency defogger.
As you can see it is still a phenomenal defogger. The other birds might be overall better but that doesn't make it any less reliable on it's own.
Now moving on to the list that you made about the mons that defeat Corviknight, Slowking G, Tapu Fini, T punch Melmetal are completely walled by Corviknight. The standard flame from slowking g is not a 2HKO and Corviknight can roost while it is threatened by stab brave bird on it's lower phys def, even max spatk Hydro from fini doesn't 2HKO and it can just uturn out to an appropriate mon. Even banded t punch from melly does around 75% which will do even less when Corviknight is roosting which Corviknight always outspeeds it unless jolly melly.
Specs lele can only 2HKO woth T bolt which it usually doesn't run. Without it, lele is walled. Against Hydreigon it can just uturn out on the plot.
Mons like Latios, Urshifu, Spectier, Nidoking, rain banded Barraskewda, Bulky Volc, Dragapult can't 2HKO without power boosting items and/or supereffective coverage moves and can't OHKO regardless. Corviknight can roost pressure stall a lot of these 1v1 and can uturn out or do considerable damage with brave bird in emergencies. A lot of these mons don't even want to switch into a brave bird to begin with.
Corviknight is not nearly as bad as you are making it seem against the current meta game. It is actually great against top meta game threats like Clef, Kart, Lando, Chomp, Torn T, Excadrill, Rillaboom, etc. While Corviknight can't win against these mons 1v1 if they start boosting but a great draw of it is that it can Uturn out before they get too out of hand not being threatened out before multiple setup turns by a lot of these.
If your running physical defensive you are screwed by special attackers, if you are running special defensive are you screwed by physical attackers
All these calcs were based on the mixed defense set that I had posted in my previous post so it is not like I am assuming spdef and phys def separately either. Corviknight does require strong and fast teammates like Banded Rillaboom, Specs Spectrier, Specs Infiltrator Dragapult to revenge kills mons it uturns out on but Balance builds should already have those kinds of mons so that is not a hinderence.
there's a lot of things that punish u-turn like ferrothorn, rocky helmet pex and moltres and zapdos.
While it's not like Corviknight loves these things, it doesn't mind taking RH nearly as much as offensive mons.
furthermore Flame Body can burn Corviknight, leaving it useless in the rest of the game.
Corviknight can still do it's job for the most part even when burned. It doesn't really dish out too much damage but it still can uturn out and defog effectively. It's effectiveness reduces, yes but calling it useless when burned is a bit too harsh and not accurate.
Lastly, a lot of people were saying that Electric and Fire types have increased in OU so Corviknight is worse but that doesn't change the fact that Corviknight still does it's job against the mons it's meant to check. Electric Types are a bad news for all the birds save Zap, which still doesn't love them. By this logic, the other birds are weak to rock which hinders their ability to check Ground types with rock coverage and defog effectively. As this is their main role, then they are simply not good as defoggers. But that is also not true. As long as you can cover it's weaknesses in builder with for example, swampert which resists both these types, gives your teams a rocker, and another momentum mon, then can play around those mons quite well. Corviknight in turn also resists grass for them. In general this simply isn't an accurate criteria. Heatran an absolute kingpin of OU is quad weak to earthquake and weak to water. These 2 types are far more common than fire and electric but it is still an amazing mon that enjoys high usage. People use ground immune mons, grassy surge mons to support it to cover this.
One thing I would like to cover is a underutilised set that I have found to be pretty cool - Max Speed and Max health Jolly Pressure Lefties, Sub Taunt Roost Brave Bird Corviknight. This set is a more offensive version of Corviknight if you will. It does not have defog utility and uturn but it makes up for it by being a win conditions on it's own. A speedy corviknight is quite deceptively fast. Outspeeding mons lacking investment like Zap, moltres, tran, lando, etc. I like this set because it can very effectively mess with defensive cores and even offensive mons don't appreciate brave birds. It has to be paired with knock off and rocks, preferably even heal bell. Spikes are also nice but optional. Paralysis support and status support could futher help the lackluster damage output of Corvi but they are not necessary either. Now if you are able to knock off some leftovers and boots it can beat some of these matchups. Magma tran completely loses its magma storms in just one sitting. Mystical fire moltres can lose it's move in 3 sittings, volt switch zap can't defeat it. Knocking off the birds is important because it can taunt on the switch or from behind the sub while they take rocks and then they can't roost or defog. If played perfectly, it can even defeat discharge zap and flame moltres off the rocks alone in 3 switches and 1 switch resp. Spikes are nice because it prevents defog with it's fast taunt against most defoggers. In general it can get free subs against, non flame clef, pex, ferro, pert, lando, etc. And then go from there. It is kind of like a baby Suicune.
I wouldn't recommend this set unless you go the complete distance of supporting it with knock off especially and some threats it can't check but if you do try it out, it is a devastating mon which can just whittle your entire team down with taunt and brave bird.
is able to run a decent specially defensive set that can check Magearna really well, and non Knock Off Clefable. Heat also comes in better on Zapdos than Wash because it resists all its moves outside rain whereas Wash takes neutral from Discharges and Tbolts. Its mostly the resistance to Fairy types that makes Heattom better than Wash, but Heattom is outclasses in most scenarios as a defensive Fire by Heatran and Moltres whereas Washtom is outclassed as a defensive Water type by over half a dozen mons.
A- => B: Ttar is a solid defensive mon, but it's also on the passive side and has a glaring 4x weakness to fighting. It walls spectrier, but other than that I think Hippowdon, which has less weaknesses and access to healing, is the better sand setter
A+ => A: Ferrothorn is still fantastic, but its lack of healing outside of leech seed means it is sometimes outclassed by walls with healing or regen. It's still an elite defensive/utility mon.
B- => C: Ditto is very gimmicky and situational. It's a decent revenge killer, but it is limited by needing scarf to avoid speed ties. Definitely not better than Amoongus, Togekiss or Mamoswine.
A+ => S: I did a post on this in the metagame discussion thread that goes more in depth about this, but to summarize, I feel that Urshifu is the most broken mon in the tier right now. The raw strength of wicked blow can power through basically anything not named Buzzwole. There's just not enough defensive counterplay
B+ => A: Lele is a great special attacker. Its got great coverage, good stabs and specs Psychic on psy terrain hits ridiculously hard. It's also got solid special bulk.
B- => A-: Glowking continues to rise in usage, and for good reason. It's not too far off to call this thing the best assault vester in the game. Good defensive typing+good coverage+regen makes this pokemon a super bulky threat that can dish out damage as well as take it.