It seems very hard to get very high defense in 5E.
Not much for a monk or most dex-based fighters or even the naked barbarian builds.
I could not see a way to get an AC above 21.
Max stat value is 20, so naked chars with 2 stats for defense can get 20.
Plate armor + shield + defensive fighting style = 21
Everything above this has to come from buffs or magic equipment, I do not know what equipment can be found.
Giving the attacker disadvantage seems to be the only way to reduce the chance to crit you below 5%.
