Once again I think Mamoswine should be considered for S-rank, this time for a very simple reason: 4 of the 5 S-rank pokemon are weak to Ice, and the 5th one (Feraligatr) is still weak to Freeze-Dry while being slower than Mamo (78 vs 80 speed). So Mamoswine either revenge kills or denies the switch-in to the entirety of the S rank, the very definition of anti-metagame. There is nothing comparable to Mamo in the entirety of the metagame and it's a welcome addition ot any kind of team except full stall. Conversely, it's very hard to find a team that isn't vulnerable to Mamoswine to some degree, as it gives trouble to offense, stall and balance.
Ice is definitely very anti-meta right now, and Mamoswine is a powerhouse that makes excellent use of its STAB combo to terrorize S-rank and the tier at large. It's got a number of dangerous sets, and you absolutely need to scout carefully so you know what you're dealing with, otherwise you might get blindsided. On the other hand, I think you're really overlooking its defensive shortcomings. Those defenses are so bad that 4/5 of those S-rank pokemon can still beat the standard LO Mamoswine 1v1 if they're at full health.
252 SpA Mega Pidgeot Hurricane vs. 0 HP / 0 SpD Mamoswine: 280-330 (77.9 - 91.9%) -- 31.3% chance to OHKO after Stealth Rock
252 Atk Life Orb Mamoswine Ice Shard vs. 0 HP / 0 Def Mega Pidgeot: 205-244 (66.7 - 79.4%) -- 31.3% chance to OHKO after Stealth Rock
252 SpA Hydreigon Draco Meteor vs. 0 HP / 0 SpD Mamoswine: 313-369 (87.1 - 102.7%) -- 93.8% chance to OHKO after Stealth Rock
252 SpA Hydreigon Flash Cannon vs. 0 HP / 0 SpD Mamoswine: 258-304 (71.8 - 84.6%) -- guaranteed 2HKO after Stealth Rock
252 Atk Life Orb Mamoswine Ice Shard vs. 0 HP / 0 Def Hydreigon: 187-221 (57.5 - 68%) -- guaranteed 2HKO after Stealth Rock
252 Atk Tough Claws Mega Aerodactyl Aqua Tail vs. 0 HP / 0 Def Mamoswine: 324-382 (90.2 - 106.4%) -- guaranteed OHKO after Stealth Rock
252 Atk Life Orb Mamoswine Ice Shard vs. 40 HP / 0 Def Mega Aerodactyl: 198-234 (63.6 - 75.2%) -- 6.3% chance to OHKO after Stealth Rock
252 Atk Life Orb Sheer Force Feraligatr Waterfall vs. 0 HP / 0 Def Mamoswine: 460-541 (128.1 - 150.6%) -- guaranteed OHKO
+2 252 Atk Life Orb Feraligatr Aqua Jet vs. 0 HP / 0 Def Mamoswine: 351-416 (97.7 - 115.8%) -- guaranteed OHKO after Stealth Rock
252 Atk Life Orb Mamoswine Earthquake vs. 0 HP / 4 Def Feraligatr: 212-251 (68.1 - 80.7%) -- guaranteed 2HKO
16 SpA Life Orb Mamoswine Freeze-Dry vs. 0 HP / 0 SpD Feraligatr: 174-211 (55.9 - 67.8%) -- guaranteed 2HKO
Note that Pidgeot and Hydreigon are much closer to OHKO'ing Mamoswine than they appear due to LO recoil. The Scarf, Band, and Sash sets are another story, but each of them has their own problems. Mamoswine is setup bait for lots of stuff when choice-locked, and Scarf and Sash need even more prior damage to get those OHKO's.
So I'm kinda conflicted about Mamoswine to S-rank. That thing sports some impressive offensive pressure and there's almost nothing in the tier that wants to switch into it, but it can't set up and falls short of KO'ing many faster threatens with ice shard. It's not nearly as formidable defensively, and this is only made worse by LO recoil. Its speed is merely okay, and it's dependent on priority or a scarf to deliver on its offensive potential. Overall, I think
A+ better suits it due to these flaws.