Just gonna throw my thoughts out on a couple things and throw out a couple more ideas.
to A+
The thing about Slurpuff is that it's quite simply not as threatening as the other S ranked pokemon in the tier, and even some lower imo. It's often predictable what set it is just looking at a team, it's sets all have a number of checks, some of which are shared between sets, and it's CM sets quite frankly struggle to hit hard until it gets a couple of cms up. Both of it's offensive sets are taken down by common priority users such as Hariyama and Kangaskhan as well. CM can be stopped by most good special tanks/walls in this tier or at least whittled down to the point where a priority move will take them out. BD Puff has a decent number of checks as well, such as Weezing and Steelix, which can easily stop it. Also, once you click BD Slurpuff isn't switching out without becoming almost useless, so if your opponent has anything that can deal with it you basically have used up your puff for maybe damage off on something or a kill at best.
It's still a great mon and can be devastating, but not one of the greatest, and not on par with mons like Mesprit imo.
to S
I really have some mixed feelings on this one. On one side, you have it being predictably choiced, getting easily whittled down by hazards, and losing it's eruption nuke if it takes any notable damage. On the other side, you have one of the most devastatingly hard hitters in the tier that can also run scarf and clean, while hitting incredibly hard. It's able to 3hko almost the entire tier at worst, and more often than not 2hko or ohko. Very little switches into a specs eruption once. There are also quite a few solid spinners and defoggers in the tier right now, such as Kabutops, Sandslash, Cryogonal, Vullaby, Togetic, Mantine, Swanna, and Pelipper. Specs Typhlosion is one of the biggest threats in the tier right now, and also having the ability to run scarf can make it hard to deal with until you know what set it is, and that often can involve either losing a mon or taking heavy damage on it. It's incredibly scary and powerful, and even if you get damage off on it to take away eruption you can knock it into the even more threatening blaze range.
I'm leaning towards S rank just because of how incredibly threatening this thing can be and how hard it is to switch into safely.
to A+
Lilligant is one of the most dangerous sweepers in this tier. Access to sleep powder gives it quite a few setup opportunities, quiver dance is an extremely effective set-up move, and giga drain giving it recovery makes it pretty hard to take this thing down with special attacks after it gets a quiver or two up. There's not a very large amount of things that stop the typical hp fire variant, and most of those lose to hp rock. Sleep Powder also makes it difficult to stop this thing with most of the mons that check it, and you are pretty much forced to sleep fodder something, quite possibly crippling that mon for the rest of the match. It certainly has flaws, such as being taken down by ice shard users like Sneasel and Piloswine, and being unable to do enough to Magmortar unless it carries hp rock, and if it carries hp rock it then becomes stopped by ferroseed who can both para it and take advantage of Lilli's speed boosts with gyro ball. However, not knowing which hidden power it carries makes sending the right thing in to stop Lilli even harder.
I feel it's definitely deserving of A+ Rank just because of how devastating Lilligant can be.
to B-
Dodrio is an incredibly powerful pokemon, with base 110 attack and STAB Brave Bird. The only things switching into CB Dodrio that well are some Steel and Rock type pokemon, which Dodrio can knock off to hinder or cripple for the rest of the match. Outside of those, however, Dodrio can devastate even some physical walls.
252 Atk Choice Band Dodrio Brave Bird vs. 248 HP / 252+ Def Pelipper: 187-222 (57.8 - 68.7%) -- guaranteed 2HKO after Leftovers recovery
-1 252 Atk Choice Band Dodrio Brave Bird vs. 252 HP / 252+ Def Qwilfish: 151-178 (45.2 - 53.2%) -- 90.6% chance to 2HKO after Stealth Rock and Black Sludge recovery
252 Atk Choice Band Dodrio Brave Bird vs. 252 HP / 252+ Def Weezing: 166-196 (49.7 - 58.6%) -- guaranteed 2HKO after Stealth Rock and Black Sludge recovery
It also easily breaks things like Tangela, Vileplume, and Gurdurr of course, and easily 2hkoes Audino with even Frustration as well. It also has it's normal stab for when it doesn't need to be taking brave bird recoil, which hits most things hard enough anyways. Plus, it has quick attack, which is able to pick off and revenge weakened mons if needed. It faces competition from the better Swellow, who isn't choiced and has U-turn and better speed, but also isn't taking recoil every turn and doesn't get whittled down quite as easily, though SR and Brave Bird is of course able to take it's toll. It however hits slightly harder with Brave Bird than even Swellow's Facade.
252 Atk Guts Swellow Facade (140 BP) vs. 252 HP / 252+ Def Weezing: 163-193 (48.8 - 57.7%) -- guaranteed 2HKO after Stealth Rock and Black Sludge recovery
It also ends up with a much more powerful quick attack
252 Atk Guts Swellow Quick Attack vs. 4 HP / 0 Def Cryogonal: 181-214 (64.1 - 75.8%) -- 6.3% chance to OHKO after Stealth Rock
252 Atk Choice Band Dodrio Quick Attack vs. 4 HP / 0 Def Cryogonal: 214-253 (75.8 - 89.7%) -- guaranteed OHKO after Stealth Rock
Dodrio even has other effective tricks over Swellow like Knock Off and Pursuit, and even has the ability to run a scarfed set and more gimmicky sets such as Sub Endeavor and WP Endure Flail.
TL;DR: The bird is strong, powerful, and has various niches.
to B-
After playing around with Vullaby for a while, I've really started to feel that it should be on par with Togetic for it's various niches. First off, Vullaby has a resistance to ghost and immunity to psychic, both of which are very useful. Second, it has taunt, which can hinder a lot of mons, such as Ferroseed and Audino. Third, it has Foul Play pared with amazing physical bulk, which can be an amazing tool as Vullaby can be able to check or countera lot of notable threats, such as Feraligatr, Sawk and even Archeops in some scenarios.
252 Atk Archeops Stone Edge vs. 248 HP / 252+ Def Eviolite Vullaby: 200-236 (58.3 - 68.8%) -- guaranteed 2HKO
+2 252+ Atk Feraligatr Waterfall vs. 248 HP / 252+ Def Eviolite Vullaby: 144-169 (41.9 - 49.2%) -- guaranteed 3HKO
+2 252+ Atk Feraligatr Ice Punch vs. 248 HP / 252+ Def Eviolite Vullaby: 180-212 (52.4 - 61.8%) -- guaranteed 2HKO
252+ Atk Choice Band Mold Breaker Sawk Close Combat vs. 248 HP / 252+ Def Eviolite Vullaby: 181-214 (52.7 - 62.3%) -- guaranteed 2HKO
(seriously why is this thing so bulky)
It's able to switch into a lot of physical attackers, of course and can also even take on some special attackers, or at least live a hit from them and dish out some damage back
252 SpA Mesprit Dazzling Gleam vs. 248 HP / 0 SpD Eviolite Vullaby: 144-170 (41.9 - 49.5%) -- guaranteed 3HKO
252+ SpA Choice Specs Mesprit Thunderbolt vs. 248 HP / 0 SpD Eviolite Vullaby: 264-312 (76.9 - 90.9%) -- guaranteed 2HKO
252 SpA Rotom Thunderbolt vs. 248 HP / 0 SpD Eviolite Vullaby: 224-266 (65.3 - 77.5%) -- guaranteed 2HKO
252 SpA Typhlosion Eruption (150 BP) vs. 248 HP / 0 SpD Eviolite Vullaby: 205-243 (59.7 - 70.8%) -- guaranteed 2HKO
252 Atk Life Orb Electivire Wild Charge vs. 248 HP / 252+ Def Eviolite Vullaby: 213-252 (62 - 73.4%) -- guaranteed 2HKO
imo this thing is one of the best defoggers as it's not mostly dead weight unlike some(*cough*Lumineon*cough*), and has enough bulk to get off crucial defogs in a lot of scenarios.
It's unique characteristics, stats, typing, and moves can make it very effective.
Also, just some quick D ranking notes
Raticate to D, it's got a couple tricks that give it a slight niche, though for the most part it's outclassed by mons like Ursaring, Swellow and Zangoose.
Fearow to D-, access to Drill Run (and U-turn in Dodrio's case) give it a niche over other flying types in the tier. Not amazing, but not absolutely garbage either. It's not like it's any worse than Wigglytuff.