OK this ended up being quite long:The important rule is: "Some spells and class features give you a different way to calculate your AC. If you have multiple features that give you different ways to calculate your AC, you choose which one to use."
DEX focused Barbarian or using +3 Plate or Half Plate armour.
Barbarian at level 20 gets Primal Might (+4 STR and CON, raises the cap to 24), so can have 24 CON.
So Unarmored Defence with DEX 20 (+5), CON 24 (+7) = 10+5+7 = AC:22 base
OR
+3 Magical Plate armour and Protection fighting Style = AC: 18, +1 Protection Fighting Style, +3 Plate) = AC:22
OR
+3 Magical Half Plate armour, Medium Armour Mastery Feat, DEX 16 or more = AC: 15, +1 from Protection Fighting Style, +3 from DEX with Medium Armor Mastery Feat, +3 magical Half Plate) = AC:22
(+3 magic armor does not require attunement)
Using a +3 magical shield
+2 from shield,
+3 from it being a Magic Shield +3. (+3 shield does not require attunement)
= AC: 27.
The +3 Shield doesn't require attunement.
Then Ioun Stone of Protection- +1 AC (Requires attunement)
Cloak of Protection +1 AC (Requires attunement)
Ring of Protection +1 AC (Requires attunement)
= +3 AC
= AC:30 (AC 35 in 3/4 cover)
If you replace one of the items with a Defender (Requires attunment) and use it's full bonus then it goes to +3, +1, +1
= AC: 32 (AC: 37 in 3/4 cover)
Contrary to initial assumptions, there are no enchantment bonus or similar anti-stacking mechanic. I believe the semi-official tweets confirm it's ok to stack AC from magic items if the DM lets you get hold of them.
Then someone casts Shield of Faith on them for +2 more AC. And SOMEONE ELSE casts Haste on them (they're both concentration spells) for another +2 AC
= AC:36, (AC:41 in 3/4 Cover) with shield of Faith and haste.
(See using Defensive Duelist below...)
For *spike* AC:
Level 17 Hunter Ranger DEX 20, Medium Armour Master and Defensive Duelist Feats and the Multi Attack Defense Hunter feature... and uh Multiclassing into Fighter or Paladin to get Protection fighting style (missed this isn't in the ranger list
AC: 22 with +1 Defensive Fighting Style, +3 Magic Half Plate, +3 DEX (Medium Armour Master). (Does not require attunement)
AC: 27 with +3 Shield (Does not require attunement)
+1 AC from Cloak of Protection (Requires Attunement)
+1 AC from Ring of Protection (Requires Attunement)
+3 AC from Defender (Requires Attunement)
= AC: 32 (AC: 36 in 3/4 cover)
+2 AC from Shield of Faith = AC: 34 (AC 40 in 3/4 Cover)
+2 AC from Haste
= AC: 36 (AC 40 in 3/4 Cover)
Defensive Duelist (When you are wielding a finesse weapon with which you are proficient and another creature hits you with an a melee attack, you can use your reaction to add your proficiency bonus to your AC for that attack, potentially causing the Attack to miss you. Then as a Reaction after (somehow) being hit by a melee, can increase AC against that single hit by Proficiency bonus: so +6 at levels 17+ putting it at:
= AC: 42 (47 in 3/4 Cover.)
(Shield spell also uses your reaction and grants +5 but lasts until the end of your turn and it's just melee. +6 AC due to proficiency.)
And if somehow you had been hit in the turn (and didn't use your reaction to stop it...)
Multi Attack defense (When a creature hits you with an attack you gain a +4 bonus to AC against all subsequent attacks made by that creature for the rest of the turn.)
So AC: 47 (AC 51 in 3/4 Cover)
So AC 51... against a single hit, after being hit by an earlier attack in that turn and using your reaction...
I Had a look at the Battlemaster using Superiority Dice, I think you can add +D12 to your AC using Evasive Footwork but the phrasing is (When you move, you can expand one superiority dice, rolling the dice and adding the number rolled to your AC until you stop moving)
Which could be (defensive fighting style, +3 plate armor, +3 shield, Defender, ring and cloak, shield of faith and cloak etc)
= AC: 36 + D12
AC: 47+D12 if you're in 3/4 Cover (as you move...) and use your reaction to activate "Defensive Duelist" upon actually being hit....
The Tarrasque (and apparently so does Tiamat) has +19 vs AC attack...
That's using the core PHB and DMG.
I've missed out: Using Manual of Bodily Health and Manual of Quickness of Action, Epic Boons or Artifacts.
(Edit, yeah apparently add +1 AC for being war forged >>. Thanks CCS...)
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