I'm looking for the best Special tanks, pretty much.
I wouldn't consider Shuckle a great Special tank due to its dismal 20 HP and 10 SpA. Here are some more balanced suggestions, which by the definition of tank can take and deal damage:
Hope I helped!
PokemonDB Stats List
Special Attack: 194 (Mega Mewtwo Y)
Special Defense: 230 (Shuckle)
OR if you want to use something useful
Special Defense: 200 (Regice)
Hope I helped