If you're looking for a Physically defensive Pokemon, then Fissure Ting-Lu has 155 HP and 125 Defense, although Horn Drill Rhydon may surpass it with Eviolite equipped.
As for Specially defensive Pokemon, Fissure Snorlax takes the cake with 160 HP and 110 Special Defense, although Ting-Lu's Vessel of Ruin may allow for it perform better than Snorlax.
However, if you want a Pokemon with the best defensive stats overall, then Ting-Lu has 155 HP, 125 Defense, and 80 Special Defense (360 total), along with Vessel of Ruin helping out its weaker Sp. Def stat.