TRC, don't take this the wrong way, but I'm going to reject Cloyster.
I used to support it, but metagame shifts along with a new standard for QC rejection have changed my opinion on it. Yes, it's quite strong after a Shell Smash boost, but (a) getting the boost and (b) getting past Cloyster's many checks afterwards is simply too difficult and warrants too much support for Cloyster to truly be effective. A poor defensive typing alongside a Stealth Rock weakness PLUS the Defense- and Special Defense- lowering effect of Shell Smash makes Cloyster a risk, sometimes a liability, to bring into battle and attempt to boost with. High defense is nice but it's literally the only cushion to have to rely on, which is barely worth anything when you're so vulnerable. Pokemon like Ferrothorn, Scizor, Rotom-W, Mega Venusaur, and Aegislash are simply far too common and far too hard to dispose of or even weaken, considering Cloyster needs them at rather low health before it can hope to get past them. Even if Cloyster manages to weaken one of those Pokemon before it goes down, there are far better Pokemon for this job that don't require such precarious setup (see: wallbreaker). Priority is extremely common, and while Cloyster resists Ice Shard and Aqua Jet, there are other ones like Extreme Speed, Mach Punch, Brave Bird, and Bullet Punch that make it extremely difficult to survive. And to top it all off, Deoxys-S and Thundurus are basically insta-death for Cloyster no matter what health it's sitting at. While Ice Shard lets it get past them, it completely loses the ability to hit many important Steel-types, thus becoming more useless than it was before.
I'm sorry man, but Cloyster's gotta go.
QC Rejected 1/3