I could be bias, but he's straight up the best dedender imo. However, his one issue is that he is amazing on top lane, but terrible on bottom. Seriously, I've never seen another pokemon rely so heavily on their starting lane, it's the difference between snow balling and being fodder. Ice shard is a god tier early game move, probably the best. High damage, long range, and freezes. It really does it all. Because of this, you can take out your teams farm very quickly and get to the enemy's farm first to use ice shard to snipe theirs. Tackle also has great range and a stun, so you can combo it very well with ice shard. Use ice shard and tackle in, and that'll take even the most tanky mons to half hp in the early game, even freezing them. If you have a slow smoke, it's straight up over for whoever you just dived. (Unless they use x speed or eject) Your objective in the early game is to play somewhat aggressively and steal the enemy's top lane exp & bees. If you succeed, the enemy has an over leveled mamoswine rushing at them, freezing, doing high damage and taking hits like a beast. With a competent ninetails, with the combined burst damage the two have and ninetail's push moves, mamo can over extend relatively safely and have nine tails bail them out if things get dicey. And their ult does extra damage to frozen enemies, so icicle crash before could be a huge help. Or with wigglytuff, both do good damage and CC, and can feel nigh unkillable at points.

Speaking of which, skillful placing of icicle crash is crucial. While a potent offensive tool, it freezes enemies if it breaks. If you space it right, you can safely retrearmt from enemies in almost any situation by putting it in a spot which forces them to wait for it to break or take the freeze, making them wait anyway. Or you can combo it with HHP and freeze multiple enemies at once, also doing a ton of AOE damage and pushing enemies.

Now, is Mamoswines ult dog shit? Not even gonna lie, it's pretty bad. But since Mamoswine excels at playing overly aggressive, being immune to CC and then stunning enemies can be great. It's nothing special, but it still does exactly what he needs, so it doesn't need to be crazy. It has definitely saved zapdos fights more than once for me by diving into a group, doing AOE and stunning.

So IMO he is great, and I think the best defender. While they aren't the tankiest, none of the others have their offensive potential and CC options. They have an easy time stealing enemy exp which can get them an easy level advantage in the early game, snowballing your team to victory.