What is the hardest to kill zombie that can spawn naturally?

Solution 1:

The Minecraft wiki lists the maximum enchantment level for zombie armour as 22 here (archive). That can at most give the zombie Protection 3 on all pieces and Feather falling 4 on the boots, according to this very useful enchantment chart.

That together with their natural armour points can make them quite resistant to fall damage. I just tested it with this command:

summon zombie ~ ~-3 ~ {ArmorItems:[{id:"diamond_boots",Count:1,tag:{Enchantments:[{id:"protection",lvl:3},{id:"feather_falling",lvl:4}]}},{id:"diamond_leggings",Count:1,tag:{Enchantments:[{id:"protection",lvl:3}]}},{id:"diamond_chestplate",Count:1,tag:{Enchantments:[{id:"protection",lvl:3}]}},{id:"diamond_helmet",Count:1,tag:{Enchantments:[{id:"protection",lvl:3}]}}]}

It can survive a 103 block deep fall, so you would have to make your drop distance 104 blocks. If you really want to do this and stand at the bottom, you can at most make your spawn platform a circle with radius 74, because outside of that radius (128 blocks away from you), nothing spawns. And that still doesn't kill baby zombies on chickens.

Fun fact: Zombies can use totems of undying when they hold them, that means that if your mob farm is in a village and you somehow make a raid spawn inside of it and an evoker dies, drops a totem and a zombie picks it up, then no fall height will be enough. And chickens. Chickens are much more likely and are also not killed by fall.

How about just using magma blocks instead?
Or lava, under that a sign, under that a slab, under that a hopper. That way you automatically collect all drops and you can make sure that all monsters are killed, except for witches. If you also want to kill witches, you could use 23 minecarts in one space, that way every mob that falls into it is extremely quickly crammed to death, too fast for witches to drink their potions.