I don't disagree with Shellos to B- because it's still probably better than most of the C+ pokes, but B seems like it's too high.
Having used both Shellos and Slowpoke in the past, I honestly find Shellos to be less impressive than Slowpoke. For all of Slowpoke's weaknesses, it can actually be quite irritating; neutral physical moves generally fail to even 2HKO it, and Regenerator allows it to wall specific Pokemon endlessly. My biggest concern with Shellos is that it simply doesn't do a whole lot; between its only average bulk for a wall and its role generally forcing it to switch into neutral instead of resisted attacks, it's generally relegated to nothing more than mashing Recover and only occasionally aiming for a Scald burn. I know I don't usually speak very highly of Slowpoke myself, but even though Knock Off is a concern, it doesn't actually instantly end Slowpoke, because two of its most important partners, Foongus and Mienfoo, are excellent Knock Off absorbers, and even without Eviolite, Slowpoke can avoid many weaker 2HKOs due to its obscene natural bulk. Slowpoke does require some support to do what it needs to do, but it still functions quite effectively and honestly, support from Mienfoo/Foongus is not very much at all to ask for at all because both are absolutely phenomenal Pokemon anyway and just about any team that can adequately handle Fletchling will enjoy their presence. I would argue that Shellos shouldn't be considered the better Pokemon, because I personally find that Shellos frankly isn't very threatening, whereas Slowpoke can actually make its presence known because it doesn't need to spend the vast majority of its turns on the battlefield using Recover.
Other than that, I would like to nominate Ponyta for A. The defensive set is nothing short of amazing; its mere presence discourages some of the biggest threats in the tier, including all three S-rank Pokemon, from even attacking. On top of this, Ponyta is the fastest Will-O-Wisp user in LC, allowing it to cripple a variety of offensive threats before they can even react, and has recovery so it can stick around and stall opponents out with burn damage. Ponyta has other advantages, of course; its excellent Speed, high base power STAB moves and offensive stats allow it to pose as a veritable offensive threat, and it's arguably the second best Sun setter after Vulpix, being able to lure in and take out Chinchou with its Sunnybeam set. However, I think its defensive potential is what really makes it great; the vast majority of offensive threats in the current metagame are physical, and Ponyta beats physical attackers. The Stealth Rock weakness can be troublesome, but when Drilbur and Archen are two of the most effective Pokemon available, and Vullaby and Staryu are both viable as well, it can certainly be worked around and the support is more than worth being able to take advantage of Ponyta's strengths.
This may be a bit more controversial, but I would also like to nominate Cottonee for A- (keeping in mind that A- is still very good). Being completely walled by one or two Pokemon is acceptable, but the problem with Cottonee is that it is easily forced out by numerous prominent threats; looking at the A category alone, there's Magnemite, Ferroseed, Foongus, Porygon, Spritzee, Croagunk, Ponyta, and Foongus. Between its low offensive stats and low base power moves, Cottonee is very weak, meaning it's really not all that scary to face at all unless your team relies heavily on something Cottonee beats. Even then, Cottonee lacks reliable recovery, and as such, is extremely easy to wear down. Cottonee is still great; its typing is still incredible, Prankster Memento is useful for getting potent sweepers into play, Prankster Encore is great for discouraging opposing sweepers from setting up, and it has an enormous disruptive movepool in general. However, it struggles to be very useful on defensive teams because it has trouble lasting long enough to make an impact, and offensive teams do not appreciate how it often detracts from momentum because there are so many safe switch-ins to it; I just feel as if it has noticeably more trouble fitting onto a team than other A-ranks.
