Not to minimod but for clarification you should probably include the typing in the bold part of your nomination.
Volcanion D -> B , at the very least C rank
So I've been messing around with and building with Volcanion. I'm really confused as to why it's D rank on water. It's much more usable than the other mons in D rank such as cloyster, politoed, and crawdaunt. It can function as an offensive defogger, water immunity, and fairy resist. It also is one of the few pokemon on water that can live a scarf woodhammer from tapu bulu and retaliate. It's also much easier to fit on teams than other pokemon in D rank, and more in line with the usability of pokemon in B rank, at the very least C rank. It can function as a breaker as well with choice specs. Using the set Fire Blast/Steam Eruption/Earth Power/Hidden Power Electric, it can plow through offensive cores such as flying, poison, and also checks belly drum Azumarill. It also naturally checks Sableye. Venusaur getting two hit koed by fire blast, or taking up to 90% from Z fire blast on offensive defogging sets, and toxapex getting two hit koed by both variants especially if its burned previously. The only argument I can assume for volcanion's "low viability" is that it fills the roll that keldeo traditionally fills as a breaker, which it doesn't do as well since keldeo has better speed, better coverage, beats chansey, hits on the physical side of special walls with secret sword, and hits water resists with the combination of hp electric/icy wind; though it lacks defog utility and is walled by grass types such as venusaur and tapu bulu. I found that I liked pairing them together though. Volcanion can do the breaking so keldeo can run cm/taunt or cm/sub with a z item while volcanion is specs. If the team is in need of a defogger, you can run Defog Z fire blast volcanion and have keldeo do the breaking. They pair really well together.
Here are some notable calcs for ya'll to chew on:
Vs Toxapex
252+ SpA Volcanion Earth Power vs. 252 HP / 4 SpD Toxapex: 158-188 (51.9 - 61.8%) -- 96.1% chance to 2HKO after Black Sludge recovery
252+ SpA Choice Specs Volcanion Earth Power vs. 252 HP / 4 SpD Toxapex: 238-282 (78.2 - 92.7%) -- guaranteed 2HKO after Black Sludge recovery
Vs Mantine
252+ SpA Choice Specs Volcanion Hidden Power Electric vs. 252 HP / 16+ SpD Mantine: 292-344 (78 - 91.9%) -- guaranteed 2HKO after Leftovers recovery
Vs Fairy
252+ SpA Volcanion Inferno Overdrive (185 BP) vs. 252 HP / 4 SpD Klefki through Light Screen: 372-438 (116.9 - 137.7%) -- guaranteed OHKO
252+ SpA Volcanion Inferno Overdrive (185 BP) vs. 252 HP / 4 SpD Clefable: 361-426 (91.6 - 108.1%) -- 50% chance to OHKO
252 Atk Tapu Bulu Wood Hammer vs. 0 HP / 4 Def Volcanion in Grassy Terrain: 252-297 (83.7 - 98.6%) -- guaranteed 2HKO after Grassy Terrain recovery
Vs Venusaur
252+ SpA Volcanion Inferno Overdrive (185 BP) vs. 252 HP / 28 SpD Thick Fat Venusaur-Mega: 278-330 (76.3 - 90.6%) -- guaranteed 2HKO
252+ SpA Choice Specs Volcanion Fire Blast vs. 252 HP / 28 SpD Thick Fat Venusaur-Mega: 248-294 (68.1 - 80.7%) -- guaranteed 2HKO
Watch volc put in work ;u;
Me vs NJNP (while he was streaming)
https://replay.pokemonshowdown.com/battlearea-gen7monotype-35863
In this game Keldeo + Volc worked in tandem to continually pressure jellicent till volcanion eventually knocks it out with its Z
Me vs Trash (Smog Exhibition)
https://replay.pokemonshowdown.com/gen7monotype-705191171
Also your calc against Bulu seems a bit confusing to me, as it looks like Volcanion can be KO'd by tge smallest amount of chip damage, including Stealth Rock. Volcanion isn't really succesful as an offensive Defogger because it's weak to SR and lacks any recovery at all on offensive sets, and as previously mentioned, SR damage causes Volcanion to fall to Bulu. But I'm not a particular expert on using Water (if I use it at all I use some combination of Swift Swim and balance) so maybe I'm wrong at this
Although I have to agree that it's kinda weird to me too that Volcanion is only D rank, it looks more like a C mon to me.