You can check the armor of a creature by unlocking the first stage of a creature in your bestiary. If you don't have that stage done it will appear as a ? and you won't know what the armor is. A way to go around this will be by checking out one of Tibia's prized fansites, Tibia Fandom. They actually have a Bestiary Simulator which displays a fully unlocked bestiary (all stages you can unlock including loot). Here's a link if your curious: https://tibia.fandom.com/wiki/Bestiary/Simulator
Based on my research and thanks to the Bestiary the only creatures we can verify as the highest armor are:
Modified Gnarlhound with 200 armor
Ogre Ruffian with 102 armor
Evil Prospector with 100 armor