Level Up (A5E) Best Martial AC, no multiclass, no magic.

[Tessarael]

A level 17 Walking Rock adept with a +3 spider-silk padded leather, a +3 heavy shield, brutal defense, and a potion of storm giant strength is sitting 16 + 9 + 5 + 6 = 36 AC. I’m sure there are some other magic items, like rings of protection, that require attunement and which could push this even higher.

This seems too good to me. As a DM, I'd be reluctant to give out Giant Strength Potions anyway, because they break bounded accuracy. So if your Strength was limited to 20, max 22 with a stronghold, your AC would be:

AC 26 = 10 base + 6 Strength bonus + 2 padded leather + 3 + 2 heavy shield + 3

I'm not clear what the Walking Rock archetype from Gate Pass Gazette #29 is, but I gather that's giving proficiency bonus to your AC. That seems rather broken to me, if so. I don't see any maneuvers in Adamant Mountain that give this.

Now you can add +1 AC from Ring of Protection and +1 AC from Cloak of Protection and +3 AC from a Defender bastard sword that gives you parry +d4 AC vs. a single melee attack each turn. Add Haste on top of that from an allied spellcaster or Potion of Speed. So these boost your AC from 26 up to 33+d4, which is about the limit of what I consider to be reasonable for Tier 4 characters. You can add a further +1 AC with Relentless Resilience from paragon Orc origin.

 

[Tablite]

No multiclass and no magic items? You just need Planestrider's Journal

• In Planestrider's Journal, the Plantfolk heritage's Bark-like gives +1/2 your Proficiency to your AC (any AC, not just unarmored). So +3 at 20th level
• The Cursed Claw Fighter archetype sets your unarmored AC a 10 + Dex + Con + shield, with an additional +1 at 5th, 10th, 15th, & 20th (+4 overall)
• Assuming you can get Dex and Con to 20 by 20th level, that's 10 +5(dex) +5(con) +4 (cc) +3(Pf) +2(shield) for a total of 29.
• If you're using a Parrying weapon, that can add at least 1d4 against a melee attack but lose the shield bonus (27+1d4)
• If you're using a Defensive weapon, that's a bonus action for +1 AC (30)
• If you can convince your DM to let you take another heritage's gifts for a mixed ancestry, Dryadborn's Protective Petals gives a +5 to your AC for a round on a reaction (like the Shield spell). That's now 27 +5 +1d4 or 30 +5

35 AC with no potions, +3 items, or spells

 

[Distracted DM]

No multiclass and no magic items? You just need Planestrider's Journal

• In Planestrider's Journal, the Plantfolk heritage's Bark-like gives +1/2 your Proficiency to your AC (any AC, not just unarmored). So +3 at 20th level
• The Cursed Claw Fighter archetype sets your unarmored AC a 10 + Dex + Con + shield, with an additional +1 at 5th, 10th, 15th, & 20th (+4 overall)
• Assuming you can get Dex and Con to 20 by 20th level, that's 10 +5(dex) +5(con) +4 (cc) +3(Pf) +2(shield) for a total of 29.
• If you're using a Parrying weapon, that can add at least 1d4 against a melee attack but lose the shield bonus (27+1d4)
• If you're using a Defensive weapon, that's a bonus action for +1 AC (30)
• If you can convince your DM to let you take another heritage's gifts for a mixed ancestry, Dryadborn's Protective Petals gives a +5 to your AC for a round on a reaction (like the Shield spell). That's now 27 +5 +1d4 or 30 +5

35 AC with no potions, +3 items, or spells

I remember that Planestrider's was REALLY busted when it was first released, I don't know if they rectified it.. I think there was something super funky with features that gave you larger stacking sizes and the benefits/rules granted by them?

 

[Verall]

I remember that Planestrider's was REALLY busted when it was first released, I don't know if they rectified it.. I think there was something super funky with features that gave you larger stacking sizes and the benefits/rules granted by them?

You may be correct in a broad sense, you can certainly stack sizes for some real shenanigans thanks to planestriders but the benefit of that is likely not broken.

You can go 5 feats into ingens to be permanently huge, but that would be at level 20,
3 feats would get you to large permanent at lvl 12.
Stack that with enlarge and rapid evolution for 2 size bumps as well as berserker behemoth for an addition 3 size bumps. There are some other options you could use in substitution.

Ultimately you can end up Titanic in size, your mileage will vary greatly on that.