Cloyster is a good Pokemon because of its typing, Ability, and movepool.
Cloyster is a Water/Ice-type Pokemon. This typing gives it four times resistance to Fire-type moves and weakness to Fighting-type moves. Its Ability, Shell Armor, protects Cloyster from critical hits. Cloyster also has access to the powerful STAB (same type attack bonus) move Icicle Spear, as well as the coverage move Rock Blast. These moves allow Cloyster to hit most of the tier for super effective damage. In addition, Cloyster can use the Moves Surf and Avalanche for additional Water-type and Ice-type coverage, respectively.