ECL 5 - Poor AC: 16 . . Decent AC: 21 . . Good AC: 26
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Achaierai (AB +9)
Air Elemental, Large (AB +12)
Animated Object, Huge (AB +9)
Arrowhawk, Adult (AB +12)
Barghest, Greater (AB +13)
Constrictor Snake, Giant (AB +13)
Devil, Bearded (Barbazu) (AB +9)
Dire Lion (AB +13)
Earth Elemental, Large (AB +12)
Fire Elemental, Large (AB +10)
Genie, Djinni (AB +10)
Gibbering Mouther (AB +4)
Hag, Green (AB +13)
Hydra, Six-Headed (AB +8)
Lycanthrope, Werebear (AB +13)
Lycanthrope, Weretiger (AB +11)
Manticore (AB +8)
Monstrous Spider, Huge (AB +9)
Mummy (AB +11)
Nightmare (AB +9)
Ooze, Ochre Jelly (AB +5)
Orca (AB +12)
Phase Spider (AB +7)
Rast (AB +6)
Shadow Mastiff (AB +7)
Skeleton, Ettin (AB +10)
Sphinx, Hieracosphinx (AB +13)
Spider Eater (AB +8)
Tojanida, Adult (AB +10)
Troll (AB +9)
Water Elemental, Large (AB +10)
Winter Wolf (AB +9)
Excluded: Basilisk, Cloaker, Ravid, Wraith, Dragons
Average: 9.8125
ECL 6 - Poor AC: 17 . . Decent AC: 22 . . Good AC: 27
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Baleen Whale (AB +17)
Belker (AB +9)
Bralani (AB +11)
Cryohydra, Five-Headed (AB +6)
Demon, Babau (AB +12)
Devil, Chain (AB +10)
Digester (AB +11)
Dinosaur, Megaraptor (AB +10)
Ettin (AB +12)
Girallon (AB +12)
Hag, Annis (AB +13)
Hydra, Seven-Headed (AB +10)
Lamia (AB +12)
Monstrous Centipede, Gargantuan (AB +11)
Pyrohydra, Five-Headed (AB +6)
Salamander, Average (AB +11)
Shambling Mound (AB +11)
Skeleton, Advanced Megaraptor (AB +9)
Tendriculos (AB +13)
Wyvern (AB +10)
Xill (AB +7)
Xorn, Average (AB +10)
Zombie, Gray Render (AB +16)
Excluded: Will o' Wisp, Dragons
Average: AB +10.826
ECL 8 - Poor AC: 20 . . Decent AC: 25 . . Good AC: 30
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Athach (AB +12)
Behir (AB +15)
Cryohydra, Seven-Headed (AB +10)
Devil, Erinyes (AB +15)
Dinosaur, Tyrannosaurus (AB +20)
Dire Tiger (AB +20)
Genie, Efreeti (AB +15)
Genie, Noble Djinn (AB +15)
Giant, Stone (AB +17)
Gorgon (AB +12)
Gray Render (AB +15)
Hydra, Nine-Headed (AB +13)
Lammasu (AB +12)
Mohrg (AB +12)
Monstrous Spider, Gargantuan (AB +15)
Naga, Dark (AB +7)
Octopus, Giant (AB +10)
Pyrohydra, Seven-Headed (AB +10)
Shield Guardian (AB +16)
Skeleton, Young Adult Red Dragon (AB +17)
Sphinx, Gynosphinx (AB +11)
Treant (AB +12)
Xorn, Elder (AB +21)
Excluded: Bodak, Destrachan, Arrowhawk Elder, Ogre Mage, Hellwasp Swarm, Greater Shadow, Dragons
(note: this level had way more than its fair share of non-melee. Even the upper levels don't have this many things that can't be expected to target AC at least sometimes. There's a few marginal calls one way or the other here, but the metric is clear - if it has special attacks or qualities tied to melee attack rolls, it's in. If its non-melee special attacks don't function every round, it's in.)
Average: AB +14
ECL 10 - Poor AC: 25 . . Decent AC: 30 . . Good AC: 35
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Animated Object, Colossal (AB +25)
Demon, Bebilith (AB +19)
Formian Myrmarch (AB +15)
Giant, Fire (AB +20)
Golem, Clay (AB +14)
Hydra, Eleven-Headed (AB +16)
Monstrous Scorpion, Gargantuan (AB +21)
Salamander, Noble (AB +23)
Excluded: Rakshasa, Couatl, Guardian Naga, Dragons
Average: AB +19.125
ECL 12 - Poor AC: 27 . . Decent AC: 32 . . Good AC: 37
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Cryohydra, Eleven-Headed (AB +16)
Frost Worm (AB +21)
Kraken (AB +28)
Leonal (AB +20)
Monstrous Scorpion, Colossal (AB +34)
Ooze, Elder Black Pudding (AB +19)
Purple Worm (AB +25)
Pyrohydra, Eleven-Headed (AB +16)
Roper (AB +13)
Excluded: Greater Basilisk, Kolyarut, Dragons
Average: AB +21.3333
(note: I had to bundle CR 14-16 monsters together here, because there's only two CR 15's that aren't dragons Unbundling now that there's more levels to see the patterns from.)
ECL 14 - Poor AC: 27 . . Decent AC: 32 . . Good AC: 37
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Angel, Astral Deva (AB +21)
Archon, Trumpet (AB +21)
Demon, Nalfeshnee (AB +20)
Werewolf Lord (AB +24)
Excluded: Nightwing, dragons
Average: AB +21.5
ECL 15 - Poor AC: 28 . . Decent AC: 33 . . Good AC: 38
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Inevitable, Marut (AB +22)
Mummy Lord (AB +20)
Excluded: Dragons. CR 15 is sparse, yo.
Average: AB +21
ECL 16 - Poor AC: 30 . . Decent AC: 35 . . Good AC: 40
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Angel, Planetar (AB +23)
Archon, Hound Hero (AB +25)
Devil, Cornugon (AB +25)
Excluded: Nightwalker, Dragons... and Greater Stone Golem, because it's more than two standard deviations away from the mean and thus unrepresentative. If a Greater Stone Golem swings, it's not going to wiff against your AC. Best just to accept that fact and move on....
Average: AB +24
ECL 17 - Poor AC: 32 . . Decent AC: 37 . . Good AC: 42
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Aboleth Mage (AB +18)*
Demon, Marilith (AB +25)
Frost Giant Jarl (AB +30)
Excluded: Formian Queen, Dragons. Aboleth Mage technically qualifies, and its spellcasting is highly anemic for its level (5th level spells when PC mages now have 9th), but it's kind of unrepresentative. As a result, I'm giving it a half weighting.
Average: AB +25.6
ECL 18 - Poor AC: 35 . . Decent AC: 40 . . Good AC: 45
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Nightshade, Nightcrawler (AB +29)
Excluded: Dragons. lawl. Sparse level is sparse.
Average: AB +29
ECL 19 - Poor AC: -- . . Decent AC: -- . . Good AC: --
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Excluded: Dragons. And you thought 18 was sparse!
Average: AB --
ECL 20 - Poor AC: 36 . . Decent AC: 41 . . Good AC: 46
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Demon, Balor (AB +31)
Devil, Pit Fiend (AB +30)
Excluded: Dragons, and Big T for the same reason as the Greater Stone Golem. He isn't a "melee threat", he's a puzzle monster.
Average: AB +30.5