Having used Mamoswine quite a lot, he's definitely an A-Rank Pokemon. Ground and Ice are great STAB's together and have great coverage. Mamoswine can take on sand teams rather well, plus Dragons are kept at their knees with priority Ice Shard. Mamoswine is, quite frankly, immune to both sand and hail damage, which is a plus. He also is a nice weapon against Venusaur and some others. Also, it shits on Ttar. Not to mention it sets up SR. Pretty solid anti-meta Pokemon with some really good qualities, A-Rank for sure.