0091 Cloyster

Ice Shard is the preferred fast move due to its better energy gains, which is necessary to take advantage of Cloyster's strong charge moves. 

Icy Wind and Avalanche both cost the same 45 energy, which makes it easy to keep the opponent guessing. The former has poor damage output but Attack debuff utility, while the latter threatens much stronger damage. Liquidation offers coverage at a moderate cost. Hydro Pump is generally too expensive.

Cloyster is a niche pick in the Great League. Its above average bulk and great attacks allow it to perform well in several neutral matchups and take down a number of large threats such as Altaria, but other Pokemon can perform the same function with more general utility

Walrein and Lapras exist, and both of them largely outclass Cloyster due to their superior stats. Cloyster's one niche over them is Icy Wind, which somewhat allows it to mimic Dewgong's role in the Great League, but it lacks the bulk that makes Dewgong threatening over there. Still, Cloyster's bulk isn't terrible, and it can definitely get off multiple debuffs.

While not as wide as before, Cloyster's niche still exists in the Master League despite its relatively low CP. It can still dismantle Dragonite and punch holes in Lugia, and on top of this it also threatens to KO other huge threats in the tier such as Groudon and Garchomp. However, Regice is generally a stronger pick for most of the same matchups that you'd use Cloyster for, unless you also have good use for a STAB Hydro Pump.