Cloyster

Cloyster's viability is solely carried by Shell Smash, which lets it outspeed the entire unboosted metagame and double its offensive stats in a single turn. To complement this, it has strong STAB options that cover each other well and a Defense stat that lets it set up in front of many physical attackers, notably tanking any hit from Melmetal after Stealth Rock, as well as from Dragonite and Mega Aerodactyl. It also has Self-Destruct, which lets it sacrifice itself to OHKO or heavily damage Pokemon that could otherwise take it on like Alolan Muk and Mega Gyarados. Unfortunately, Cloyster's poor offensive movepool utilizes its lower Special Attack stat and leaves it unable to meaningfully damage Water-type Pokemon without using Self-Destruct, which can make handling it quite easy. Its abysmal special bulk limits its setup opportunities, and it faces competition from Omastar, which is vulnerable to revenge killing from the metagame's fastest Pokemon due to its poor Speed but is much more powerful and can even support its team with Stealth Rock.