Level 100 Statistics (see level 5, 50, 100)
This is the standard Cloyster set, and it does its job well. Salamence, Metagross, Flygon, Gyarados, and others generally respect Cloyster enough to switch out against it. Spikes and Rapid Spin are the obvious utility moves, as they take advantage of those switches to set up. Surf is generally preferred as a STAB move over Ice Beam to hit Metagross, Tyranitar, and opposing Forretress. Ice Beam has room if you desperately need an answer to Salamence and it can hit Celebi and Zapdos switching in twice as hard, but Surf usually has more purpose over time. The final slot is generally Explosion, as Cloyster can often live an attack and strike back to put a Pokemon down. Ice Beam can be used for the coverage of both STAB attacks, while Rest allows Cloyster to act in a more passive role, effectively walling Skarmory, Claydol, and others throughout the game. When paired with cleric support, Cloyster can be a hard shell to crack (pun unfortunately intended).
Max HP and 148 Special Defense EVs means Cloyster will never be 3HKOed by other bulky Water-types. 88 Special Attack EVs give Cloyster enough power to 2HKO uninvested Tyranitar while still having a good shot at 2HKOing max HP Tyranitar. It also lets Cloyster 3HKO physically defensive Forretress and Skarmory. 4 Speed EVs get the jump on Skarmory, allowing you to Surf on it and then spin the entry hazards away.
Team Options & Additional Comments >>>
There really aren't any other moves Cloyster can make use of, but Cloyster can use a much more physically defensive set in order to better counter threats such as Salamence. Clamp and Explosion could be used to trap something and kill it, but this is gimmicky. Hidden Power and Toxic are available as mentioned before.
Checks and Counters
Although Zapdos and Celebi must be wary of STAB Ice Beams, they can switch in with relative ease to threaten it out with their respective STAB attacks. Cloyster's STAB Surf 2HKOes and 3HKOes offensive and bulky variants, respectively, but Gengar can come in on attempted Rapid Spins and force Cloyster out with Thunderbolt. Other special attackers such as Raikou and Jirachi work as well. Calm Mind Suicune is another special sweeper that can set up on Cloyster; Substitute Suicune can even shield itself from Explosion, whereas fully defensive Suicune tanks the same move with roughly half of its health still intact.
Starmie is one of Cloyster's best counters, as the latter can't do much to Starmie while it removes Cloyster's Spikes. Watch out for Explosion when switching into Cloyster with Starmie, though. Forretress can take a hit and use Rapid Spin, but Cloyster will win one-on-one. Cloyster has to be careful about switching into too many attacks, as its lack of reliable recovery will be a pain for it.