This ones a hard one, but I'll try my best.
My first option is probably Ferrothorn.
The reason why I chose Ferrothorn is because of its moves; its main ones being Spikes/Stealth Rock, Knock Off, Leech Seed, And Thunder Wave. These moves paired together can be phenomenal on Ferrothorn as it makes it amazing for a defensive wall.
My second option is Reuniclus.
The reason I chose Reuniclus is because of its amazing base stats; 110 HP, 65 ATK, 75 DEF, 125 SP.ATK, 85 SP.DEF, and 30 SPD.
You can easily boost both of its Special stats using Calm Mind.
Use Recover to stall out your turns.
(Optional) Use Trick Room to give you a speed boost.
Spam Shadow Ball after using Calm Mind a few times.
Use Protect when you don't have anything else to use, or you just feel like stalling out your turns.
Toxapex is amazing too, the reason why I chose Toxapex is because...
Its defense stats are phenomenal, coming in at a base 152 DEF, and 142 base SP.DEF.
Some moves you could pair with the Toxapex is..
Scald
Recover
Haze
Knock Off
Toxic
Baneful Bunker.
Hope this helps!😁