Mamoswine is definitely A-Rank material. It has so much going for it, that the positives greatly outweigh the few negatives, such as his low speed or bad defensive typing. Ice/Ground hits half the meta game for super effective damage, and in this Dragon/Steel infested meta game Ice and Ground moves are really useful to have. Although Mamoswine is easily out sped by a majority of Pokemon in OU, Mamoswine has access to STAB priority Ice Shard, which stops Moxie Mence, DD Dragonite, RP Landorus, Breloom, and even Haxorus dead in their tracks. Earthquake hits Tyranitar before it can KO back with Stone Edge, and Air Balloon Heatran still has to worry about Superpower. As others had said, he's also a great Stealth Rock setter and can be an outstanding Endeavor Focus Sash lead, since he can KO back the next turn with Ice Shard. Thick Fat makes him neutral to Fire attacks, which even lets him avoid the OHKO by Timid Heatran's Fire Blast. He's great on either Hail or Sand teams since he's immune to both, and often times he is the glue of many teams since he can single handedly check almost every Dragon type in OU and provide outstanding coverage against other threats such as Breloom, Jirachi, and Landorus. Honestly, I don't think I've ever made a team that doesn't have a Mamoswine weakness. He's just that threatening. Scizor and physically bulky Waters such as Politoad, Vaporeon, and Rotom-W with Levitate are one of the few decent answers to Mamoswine. Heck, even Standard Jellicent fears an EQ from him. All in all, the goods heavily outweigh the bads, and Mamoswine seems to fit in A-Rank just fine.