Many opinions on the current list.
Slowbro: I have used it serveral times and while it is really good, is not S rank worthy. With the introduction of Assault Vest, it can finally be a mixed wall and can choose between physically defensive and mixed wal sets. However, it doesn't fair well against top metagame threats. S rank Chandelure, Houndoom, Hydreigon all murders it. Things that it check, like Damanitan, Victini, Mienshao etc, usually spams U-turn and it cannot effectively damage them without hazards (same for them though). It is hurt by the Knock Off buff since it can actually take on some of its users originally. Moreover, despite its SpA stat and movepool, Slowbro simply doesn't hit hard enough. For example, Ice Beam doesn't even come close to OHKO on Haxorus and leaves it with 4-5 turns of LO hp to spare. In return, LO Outrage has a chance to OHKO even physically defensive after rocks. T-wave varients need to watch out for Lum Berry. Slowbro's problem is that it cannot both take hit well and retaliate hard at the same time, allowing the opponent not even caring about 'super effective' move and set up on crucial times, while it lacks the sheer bulk to take on boosted opponents. This sounds more of A+ Rank for me.
Chesnaught: This thing is really overrated. Its only niche in the tier in fact is checking Crawdaunt (and other Knock Offs). Spikes seems cool until you realize that it is a magnet to all hazard removers (Mew, Latias, Zapdos, Crobat and even Mega Blastoise thanks to its inability to tank an Ice Beam). More than half of the top meta threats like Chandelure, Hawlucha, Houndoom, Victini, Kyurem, Nidoking etc eats it alive while most of the others are things it can't switch in on (Hydreigon, Slowbro, Mega Blastoise, Mega Manectric, Keldeo). It is god damn awful if Knock Off is banned. B- Rank for this shit.
Chansey: Chansey is something that is really good now. Knock Off is not an issue thanks to its users being really obvious and Chansey has no business staying on them anyway. With Wish+Softboiled (seriously, don't use downright inferior WishTect), it can even act as a pivot for balanced and even offensive teams. It is no longer a momentum loser since it doesn't need two turns for healing while being able to pass huge Wishes to check to opposite switch ins. The amount of things it walls which includes 99% of the special attacker and 60% of the physical attackers doesn't even needs mention and its incredible support movepool should be well-known. The only thing stopping it from S is it sort of gives set up opportunities to some physical sweepers and need some team support to take on those. A+ pink blob for sure.
Mienshao: Mienshao has recived a huge buff this gen on Knock Off. No longer will it be walles hard by ghosts or psychics to an extent. As oppose to what people say, Knock Off doesn't lack power. In fact, there is a high chance for Physically Defensive Mew will be 2HKOed by Knock Off + U-turn, making it insanely hard to switch into. Personally, I enjoy using a SubPass set with High Jump Kick and Knock Off. It is really easy to set up subs thanks to Mienshao's offensive pressence and can easily pass to teamates when against counters (they are rare anyway). Damage from Sub and LO are compensated by Regenerator, which is an incredible ability on Mienshao. There is nothing actually safe from Mienshao as everything is 2HKOed by its sheer power. Even extemely bulky mons and resists like Suicune, Heracross and non-physically defensive Florges are 2HKOed by LO High Jump Kick after rocks. If Reckless is taken into account, those 2HKOs doesn't even need rocks while even physically defensive Florges can fall to 2HKO after rocks. Its scarf set is stil incredibly good and gives offensive teams huge pressure. Fraility is an issue, but it hasn't stopped it being S rank last gen for a long period. Mienshao has just got more powerful than before and isn't that seriously hurt by tier changes. It should be A+ Rank.
Gardevoir: In fact, this should be A+ Rank. It is like Scarf Emboar in RU, being the most anti-meta revenge killer. It can switch in and revenge unboosted Hawlucha, Hydriegon, Latias, Keldeo, Heracross, Coil Zygarde, Band Haxorus, Crobat , Azelf, Mienshao etc with its dual STABs alone. Its extensive offensive movepook containing Thunderbolt, Shadow Ball, Focus Blast, Signal Beam allows it to tailor its set to fit the team's need. What makes it really great though, is its access to Trick and Healing Wish. With Trick, it is a scarfer that isn't deadweight against stall, crippling Chansey, Florges which usually walls it easily. Healing Wish is an incredible move for HO teams, fully recovering a sweeper while turn the momentum to the user's side. Trace is an incredible ability which is less valuable now with Drizzle and Thundurus-T banned. Gardevoir isn't even that one sided as is can run a defensive and CM set easily, which is not outclassed by Florges thanks to access to WoW and Psyshock respectively. The only thing holding it back is its lackster physical bulk and average 80 base speed.
Venomoth: I have used it extensively in UU. While I don't agree that it deserves A Rank, it shouldn't be as low as B-. While Venomoth's bulk is bad, its nice typing gives it resistance to commonly choiced fighting and bug moves. It usually can find opportunities to set up against offense while stall is even harder to stop it thanks to Wonder Skin. The only thing that can stop it from QuiverPassing is strong physical scarfers faster than it as Scarf Keldeo can't manage to 2HKO it after a boost.. However, this is its main problem as those scarfers are really good and prevalant in the tier. Victini, Darmanitan, Mienshao, Jirachi are the best scarfers right now and fucks up Veomoths which doesn't immediately bp away. While it needs team support to deal with some scarfers, it can do its job most of the time and the reward for QD passing is huge. It deserves to be B/B+ Rank
Shedinja: ...It walls a grand total of 4 offensive pokes under A/S (Keldeo, Latias, Zygarde, Kyurem)... It would be 1000X better in Ubers, where it still suck... Can't see any niche at all to be even at D Rank. Unranked/Give it a F(ail) Rank please.