Alright, Buckle in folks, because this is going to be a long post :D
Apologies if any of my suggestions / notes have already been covered or are trivial and not worth discussing.
UR = Unranked
(Water)
Golisopod | C ---> B: An incredible asset with it's powerful priority, Golosipod offers powerful Bug STAB in First Impression and Leech Life and can carve Dark, Psychic, and Grass teams to pieces. Golisopod also sports a bulky ground resist and can even set spikes using the switches it forces. Thus, I think it's more in line with the B-ranked mons.
Araquanid | B ---> C: This Pokemon's one-dimensional offenses are really transparent on water: it's also very slow and contributes little to the type defensively with unremarkable typing and unreliable recovery. Definitely a C, if not D ranked pokemon, despite it's powerful water STAB.
Slowbro | B ---> A: Slowbro is as dependable as ever as a physical wall, despite new additions like Toxapex. It's markedly less passive than Toxapex, for one, with a wide variety of coverage moves and crippling status to choose from. I think it still deserves an A ranking on Water.
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(Normal)
Drampa | C ---> B: Drampa is underwhelming, but it's still an excellent win-condition against some of the best types right now (electric, psychic, water) by abusing Calm Mind, Roost, and Berserk Boosts. Drampa boasts much more offensive presence than your typical Normal-type and has a movepool as wide as the horizon. It's not perfect, but it deserves a B ranking alongside other underwhelming additions like Bewear.
Silvally | UR ---> D: Silvally normal is another underwhelming pokemon who epitomizes Gen 7's motto: "it's a shame that...". Nonetheless Scarf Parting Shot is worthy of some recognition on Normal, with it's ability to debuff opponents to soften blows against Staraptor, Porygon2, and Chansey. Unfortunately, it can't do much more than that, so a D (maybe C) ranking is apt.
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(Electric)
Galvantula | D ---> C: Sticky Webs alone should be enough to elevate Galvantula to C-rank IMO, given the highly offensive nature of Electric Teams this gen.
Rotom-Mow | UR ---> C: Rotom-Wash is generally superior to Mow, but Mow offers useful Grass STAB to help deal with Water and Ground team's respective immunities more effectively. It is immune to Leech Seed, which can help when dealing with pesky Ferrothorns. It's not much, but it's enough to warrant some ranking.
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(Ghost)
Golurk | A ---> B: I think with the addition of Alolan Marowak, Golurk has found itself outclassed on most fronts. Since Marowak hits significantly harder and has more useful resists (Fire, Fairy, Grass, Ice, Steel), Golurk's only real defining feature is Dynamic Punch; and even that was indirectly nerfed this gen with the reduced confusion chance. Rocks resistance is handy, but I think Golurk is better suited to B-ranking now.
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(Ice)
Cloyster | B ---> A: While playing around with Aurora Veil and set up, I found Cloyster was a far more potent abuser of screens than Sandslash or Beartic. With Screens Cloyster has much more freedom in item (not having to run Sash) and can run King's Rock or Life Orb or even Z-moves. I think Cloyster's blossomed into an A-rank threat.
Beartic | A ---> B: I was hugely underwhelmed with Slush Rush Ice Offense; i feel it doesn't outperform pre-existing offensive Ice Builds. But while Alola-Sandslash offers a plethora of resists, ground coverage, and rapid spin; Beartic is slower, frailer (on special side), and is all around crap. Even with Slush Rush, I believe Beartic is a B-rank at best.
Articuno | UR ---> C: Articuno struggles to find a place on Ice teams in gen 7 with their heavy offensive bent, but as Ice's premiere Defogger, Fighting Neutrality, and Cleric, Articuno is definitely still worth ranking a C, or potentially D.
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(Grass)
Virizion | UR ---> C: Virizion is still worthwhile for all the same reasons as Gen 6. It's significantly faster than Breloom, can hit relatively hard with Close Combat and Stone Edge, and offers taunt/mixed utility. Put all that together and you get a solid C or D rank Mon.
Tsareena | UR ---> D: Rapid Spin, U-turn, and an immunity to Priority make Tsareena worth mentioning in the Rankings for it's use against Dark and Ice.
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(Rock)
Gigalith | C ---> D: I'm not sure why this pokemon is even ranked at all. SUre, it got the ability Sand Stream, but there's no good reason to use it over the far superior Sand-setter Tyranitar. I don't see any niche appeal to consider with Gigalith.
Golem-Alola | C ---> B: Alolan Golem's capacity to trap and dispose of annoying Steel-types like Skarmory, Ferrothorn, and Scizor warrants the mon a solid B-ranking.
Archeops | UR ---> C: Decent special offense, Flying STAB, blazing speed, and access to U-turn and Defog make Archeops worth a mention on the Rock Monotype Rankings.
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(Fairy)
Clefable | S ---> A: With the mounting power creep and general shift to offense this generation, Clefable isn't as effective in the metagame as it once was. Clefable is still excellent, whether utilized as a rocker, a Calm Mind Win-condition, an Unaware set-up counter or some combination of the 3. But it's not a S-rank anymore, I feel: it's not as meta-defining and essential to the type as the others of that rank.
Ninetales-Alola | C ---> B: While Alola-Ninetales may not seem to offer Fairy much, powerful ice STAB and Weather Disruption gives Fairy a large advantage against Ground Teams (and SS water) while serving as a strong lead and Screens Setter. I feel these attributes put Ninetales on similar ground as other B-ranked mons for Fairy.
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(Dark)
Incineroar | UR ---> B: Incineroar is the only Pokemon on Dark who can handle a rampaging Magearna, which is valuably to start with. But Incineroar is all around a good bulky defensive check and pivot with a neutrality to Fairy and Bug on top of many other useful resists. A solid B-rank pokemon.
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(Flying)
Gyarados | UR ---> B: Gyarados-Mega may no longer be usable on flying, but Gyarados is still a meaty ice neutrality with a lot more offensive presence than Mantine. With a wide variety of attacks, utility, and set up; Gyarados is a cozy B.
Togekiss | A ---> B: Togekiss isn't as great a Special Wall as it once was, now competing with not only Zapdos but Mantine as well. It simply isn't as effective in the current meta-game, especially with so much Steel afoot. Stallbreaker sets may have some use, but Delta Airlines isn't worth A rank anymore.
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(Psychic)
Wobbuffet | UR ---> D: Wobbufet isn't amazing, but it's probably worth mentioning for it's capacity to trap a broader range of threats than Gothitelle to remove checks for sweepers/cleaners like Mega-Slowbro, Mega-Metagross, Victini, Happy-Hour Jirachi, etc.
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(Poison)
Salazzle | C ---> B: As much as I dislike Salazzle, it's not a C-rank. Salazzle's handy fire STAB allows it to check troublesome pokemon for poison, namely Mega-Metagross and Celesteela. It's also capable of crippling defensive threats like Heatran, Toxapex, Mega-Venusaur, and Celesteela with Toxic, thanks to it's ability Corrosion (though doing so without taking heavy damage is tricky). It's worthy of a B-ranking.
Nihilego | S ---> A: This might be a contentious opinion, but I don't feel Nihilego is S-rank worthy on Poison. It's strong, versatile, and offers the type a lot of bulk and offensive presence. But it isn't essential for a Poison team, facing stiff competition from Gengar, Salazzle, and the Nidotwins. Nor is Nihilego as Meta-Defining as Mega-Venusaur or Toxapex. A-rank is far more suiting, IMO.
Dragalge | B ---> C: Dragalge offers little to Poison: Toxapex surpassing it as a T-spike setter, Phazer, and Fire resist. It can Wallbreak, but Drag's woeful speed and impotence against Steel-types make it difficult to support or abuse. It's more fitting as a C-rank.