Magmortar is shit in OU please just don't.
Alright. Mamoswine is not A in this meta. It is a good Pokemon, no doubt. Strong EQ, SR, and STAB Ice Shard are all very big pros. However, the meta has become less kind to it. With threats such as Manaphy, Kyurem-B, Keldeo, Mega Charizard, etc. very common, Mamoswine is finding a hard time. The Ice Shard-weak offensive threats are now bulkier: Pinsir can take one (or just +2 QA it to death), Zard X is a Dragon not weak to Ice, YacheChomp is common, Latias/Latios can take one...I understand it is an RKing tool, but these Pokemon are common threats that can take an Ice Shard and continue to sweep. Pokemon like Talonflame, Balloon Exca, Rotom-W, Greninja, etc. just make its life really hard, bringing it down this gen. A- at best, I see B+ as reasonable.
Also...Tyranitar in A? It is top-tier atm, with numerous viable sets. The DD Mega set is one of the most threatening OU sweepers atm, and AV a sturdy special tank that handles many top-tier special threats. Besides these two sets there is SR tank w/ Lefties or Chople, good old hard-hitter CB, and Scarf. Smooth Rock Tar is also very good to support Sand Rush Exca and Sand Force MegaChomp/Lando. Tyranitar's versatility, strength, and utility make it a clear A+ in this metagame.