Corviknight B+ > A-
Firstly, it is a fantastic defogger. It can directly switch in and defog against common rockers with it's pressure unless they are running a set to beat it in particular. For instance offensive rockers like SD Garchomp and Lando can get by all the other defoggers with Stone Edge. Garchomp can only hope to beat Corviknight with SD Fire Fang (Non boosted fire moves or even boosted Stone Edges can't get through it) and Lando with Smack Down. The former mostly can't afford it and the latter is niche. If they just start to SD multiple times, then Corviknight can even be threatened by Stone Edges but you can just Uturn at that point and they won't have the luxury to do that again after the damage from first encounter. Defensive Defoggers simply get stalled with pressure in the long game unless they have their own pressure mon. Lastly Heatran and Nidoking with Flamethrower and rocks are indeed problematic. Heatran is especially difficult but it is just one mon and is a notoriously good rocker which gives just about every Defogger grief with Toxic, Magma Storm or Taunt while the latter is rare, has no longevity and is frail. In a longer game Nidoking can't keep rocks up.
Secondly, it is a great pivot /glue mon. It is able to check a large amount of top tier offensive threats such as Melmetal, Kartana, Excadrill, previously mentioned Garchomp and Landorus, Rillaboom, Latios, Tapu Lele, Scarf Spectrier etc. While some of these mons can bust though with a lot of setup or a supereffective choiced move, the beauty of Corviknight is that it can just Uturn out before they are able to reach such levels of power and go into the appropriate revenge killer. In general, Corviknight is incredible just in pivoting around dangerous choice mons or strong mons in general. For instance, even against mons like Nidoking or Tapu koko it can come in on non-fire/ electric move and pivot or hard switch out into the appropraite mon. Other mons that Corvi can help pivot around include, mosa, specs magearna, NP torn, etc. This pivot ability makes it much less passive then it might look and it still is not a pushover when it comes to damage. Brave bird can do chunks to even bulkier mons like Kart and Lando. This mixed defensive ability is amazing and not many mons possess it. Checking Specs Lele and Banded Kart in the same game is an achievement within itself. One EV spread I would like to base some calcs off is this one: EVs: 248 HP / 84 Def / 176 SpD with any defense boosting nature and leftovers. This set notably survives +2 LO Jolly Fire Fang from Chomp after rocks 96% of time. It survives +2 LO Jolly Kart Knock after rocks always and it can survive LO Modest Mosa's Ice beam into Shock Wave or QD into Shock Wave after rocks however it can't survive Shock Wave into Shock Wave or even QD into Focus Blast, though. All the above mons I listed, were based on this set in particular. Another crazy thing it can do is that can PP stall even banded Urshifu or Specs Spectrier 1v1 but on switch ins, it does get 2HKOed.
Because of this pivot ability against such a large part of the meta game it pairs exceptionally well fast choiced mons like Spectrier, Latios, Mosa, Urshifu, Rillaboom etc. bringing them easily on mons like Clef, Fini, torn, Ttar which can be difficult to switch dirctly into.
These are my major points in favor of Corvi. Other defoggers like Mandi, Moltres, Zapdos and Torn are all afraid of Knock off and Toxic, moves which some rockers make use of and thus resulting in them beating these defoggers long term. They either don't have pressure/ don't run it and all can't dare Defog against SD Edge Lando or Chomp. These factors make it so that Corvi is arguably the best, most consistent defogger in the tier. None of them can fill the mantle of slow pivot either. They are fast pivot. Also these mons simply can't check both ends of the spectrum. Mandi has to Spdef to check Spectrier and Zapdos or Moltres simply can't take Psychics or Dark Pulses because they don't have the luxury to split defenses. Torn T doesn't have the luxury to run any defense because it needs it's speed. Magnezone is the main hinderance to Corviknight but that has not picked up enough usage to directly threaten Corvi's viability as a reliable defogger and you can switch to Shed Shell if it becomes too popular. One thing that these birds do have over Corvi is better offensive presence be it with knock off or foul play or Nasty Plot or coverage or just higher offensive stats. Also as I previously mentioned that these birds don't run pressure because they have even better abilities in the 30% contact abilities which give them ability to status other mons along with even some moves they have which Corvi completely lacks.
But Corviknight is not exactly a sitting duck. This brings us to the last point. Corvi can run a mean Sub Bulk Up set. This set is pretty effective in the current meta game. The only Hazer is Pex and it faces some competition from mons like Fini, Bro and Pert, mons which a lot of teams use as their water (A knocked off, burned/paralysed pex can be defeated anyways) and the phasing is not that common with Hippo and Garchomp carrying it seldom and Skarmory being rare itself. This leaves it the opposing team in a situation where they have to take it on head on. It completely feasts on balances that have core like bro, clef, mandi etc. Tran which was a great counter to the standard set suddenly becomes set up bait once it's 4 magma storms are depleted against sub roost. You have to knock off to beat it however if taunt so you can repeatedly chip with brave bird. Moltres and Zapdos, 2 fellow birds are admittedly a pain and have to be toxiced but if they are the only counters on the team, Corvi can simply pressure stall repeatedly and eventually take the game away. Volt switch Zap and Mystical Fire Moltres don't even stand a chance. It can even chip down down mons like specs Koko, Trick Latios, Cind with Brave Bird etc if it doesn't want to set up against them. This set is very uncommon in the current meta game because it loses 2 of it's best attributes in being a pivot and defogger and it now requires support instead of providing it. This set is, thus, not the most easy to use spashable mon to use but on the right team with sufficent support like Heal bell, mons to cripple the above mons and be a good enough defensive backbone so that Corvi can take it's time to win, then it truly shines like no other mon could (Except Mag but we won't talk about that).
If waiting to set up is not your cup of tea, you can cause a lot of disruption with sub taunt . If you just want to beat pex and hippo harder taunt bulk up is also an option. Body Press Iron defense allows you to beat all physical attackers under the sun. All these are the things it can do but honestly the first 2 sets are the best. Even more honestly the first set is the only set you will run 80% of time but these last sets have that surprise factor that makes them deadly. It is hard to pass up such a good defogger/ pivot but that sub bulk up set is not to be slept on. The point of all other sets is that it can do something other than just defog and Uturn. These bulk up sets in particular give it actual power. These sets are not nearly as consistent as the first one and lose it's best attributes but they have a lot of surprise and game winning potential. They have to be accounted for too.
Guys this was my first post I made about a ranking to be changed. I really don't know if I followed the correct format or if this was too long but I was able to somewhat say most of the things I wanted to about it. I am not exactly a wizard with words but I hope I was able to make some points worth considering.