In a similar vein to the above post there are 3 mons that stand out to me as potential defensive beasts (pun intended). These are inheritors only.
Nihilego: Nihilego's main flaw right now, besides its crippling weakness to ground, is its awful defensive stat. Couple it with something like Groudon, Arceus, Giratina, or Lugia and you suddenly have the mixed bulk mon of the century.
Buzzwole: Basically the same as above, but on the special side, and the crippling weakness is to flying. Off the top of my head good gods would be Ho-oh, Lugia, Kyogre, Giratina, or Arceus.
Toxapex: BAN THIS PLEASE. On a Giratina team it is capable of getting mixed defensive stats of 150/152/142, becoming bulkier than the god itself. Even without Giratina there are plenty of ubers with 120+ hp. It's untoxicable, has regenerator, has no crippling weaknesses, can set Toxic Spikes... like oh my god this thing will be a pain in my butt-ocks unless it gets banned and be basically impossible to break without a dedicated stallbreaker. Not just a stallbreaker but one that specifically can beat Toxapex.
NB: I'm not calling for an immediate ban of Toxapex but rather for it to be put on the radar.
PS:
Why Nerdy posted an Araquanid set above while I was typing this...
252+ SpA Choice Specs Water Bubble (Deo-A) Araquanid Surf vs. 252 HP / 12 SpD (Giratina) Toxapex: 226-266 (44.8 - 52.7%) -- guaranteed 3HKO after Black Sludge recovery....... WALLED, even without significant special defensive investment (without that 12 spd it had like a 1% chance to 2hko... which is a testament to Araquanid's power as well)
finally...