Avalugg. Cloyster has better things to do like use Shell Smash. Cloyster also doesn't pack Recover or Sturdy like Avalugg does, so it's better off being offensive Shell Smash rather than a defensive Pokemon.
Avalugg @ Leftovers
EVs: 252 HP, 252 Def, 4 Atk
- Gyro Ball/ Stone Edge/ Rapid Spin
Having said Avalugg, it has a very poor typing to wall. There are better options like Landorus-T, Slowbro, Tangrowth et. al.