Best tank is kinda moot by definition.
I guess, one that can take most damage without dying?
This is my third last play-through on solo potd 2.0 where you cant recruit, flee from combat.
In solo mode I found str to be quite useless.
It dosn't really matter from my experience what class you run since all classes can wear all armor.
Especially with the abraham pet where the heavy armor penalty is reduced alot.
This guy just don't die.
This example is a herald with 21-23 resolve, 20 con, 18 int and 8str, 8dex, 8 perception ( with items, there is no beraths blessings avaliable)
Items like ring of solitary wonderer is acheivable by just buying some perception items, maybe buy a drug or sleep with the old hag in dunnage.
Fight starts with queue actions (brand enemy on everything, chant: come come, swift of death (raw damage ingores armor) does same damge vs lv1 enemy as lv 25 enemy).
Character just stands there and let summons grind the rest. Most enemies just die from brand enemy+chant aura (ticks for about 7 damage per second when speeded up).
With Scadu Scalth shield (From old city) you get deflection from athletics and % damage reduction from metaphysics. So he kinda gets into an iron turtle.