I agree with
mdvd - Tsareena and Decidueye are pretty mediocre at their jobs and a B- ranking reflects that very well, while still acknowledging that if used effectively they can be valuable to a team.
I also agree with
LORD HELIX 65 - Crobat really does not belong in the C ranks because its great speed means it can do /something/ every game, and in several games do a lot, especially with not many Flying resists on teams. I'll add that my stall teams struggle badly with NP Crobat because its immunity to poison, instant recovery, and power after several boosts, although that is obviously not Crobat's main niche.
Two nominations of my own: Alomomola from B- to B and Poliwrath from unranked to C
On Alomomola, I nominate it to rise based on the merits of its Wish Protect set which usually runs Scald and Toxic. The benefits of this set is that is provides a very valuable support move for defensive teams, counters top threats including Sharpedo, spreads status with Scald and Toxic to whittle away, and can abuse Regenerator switches to pivot or even save PP.
Wish is almost mandatory on stall teams, for Pokemon like Tentacruel and Nidoqueen which are good defenders but have no reliable recovery. Alomomola partners with these Pokemon well because it can pass large wishes and they can switch into moves Alomomola does not like - stronger special moves or Electric / Grass moves.
The metagame is not unkind to Alomomola as potentially big threats like Celebi and Bisharp are often sidelined in favor of things like Mew - which while Alomomola cannot counter it, Alomomola can still target them with Toxic. Additionally, Alomomola can switch into essentially every single choiced attacker in the tier as long as they do not have super effective STAB moves, in order to scout. Even things like Specs Hydreigon have trouble OHKOing and frankly, that set is rare.
Alomomola's main two weaknesses - passiveness and average special bulk (for a wall) - does not badly affect its role because those traits are to be expected in stall teams and can be covered by teammates. The fact that Alomomola is limited to slower playstyles and has little diversity (maybe the occasional Knock Off or Soak?) is reflected in the fact its not being considered for the A rankings.
On Poliwrath, I nominate it to rise based on the merits of its Scald Circlethrow Resttalk set which usually maxmizes defense. This is far more uncommon than Alomomola so I will include a couple replays. The benefits of this set is that it provides a status absorbing wall that by virtue of its typing walls many top tier threats such as Scizor, Bisharp and nonboosting Keldeo.
See this game for evidence that the metagame is not unkind to Poliwrath - the Pokemon it counters are common. Unfortunately my opponent outplayed me but this game still shows the Pokemon Poliwrath counters are used in UU.
http://replay.pokemonshowdown.com/gen7uu-556360405
See this game for evidence that Poliwrath can be used on stall teams as a backup defender - even though I misplayed and let it die, it did a good job of breaking Scizor's Lum Berry and in better hands it probably did not need to faint. Also I made mistakes at the end letting Tentcruel die, but that's not really relevant to Poliwrath.
http://replay.pokemonshowdown.com/gen7uu-556356796
What these replays did not show was the utility of a RestTalk user - these Pokemon can easily absorb status and with Rest, PP stall for a long time. These traits, along with Scald's natural status spreading qualities, give Poliwrath a surprisingly useful role in combating defensive teams - walls without high quality utility moves are usually deadweight against stall but not so for Poliwrath
Unfortunately Poliwrath suffers from several problems - firstly there are several Pokemon in the tier that can abuse him like Mew and Primarina with few drawbacks. Although that is to be expected of a defensive Pokemon, the sheer number and viability of Pokemon that can switch in makes it impossible to use on any team that does not require a single counter to the mentioned threats. And Poliwrath has only above average bulk with limited recovery, so it relies a bit more heavily on team support than a typical core defender. Overall these problems mean Poliwrath has trouble justifying itself in any rank above the Cs, but C or C- is not really asking too much.
In summary, both Alomomola and Poliwrath should rise because they are strong defenders and their flaws are reflected in their still relatively low rankings and the fact they often have teammates covering them.