Seems to be some discussion on AA builds for 20 Ranger vs 6 Ranger/Fighter 12
So I wanted to see how Kensei III would compare - note tied mostly to rapier and longbow
Will be using Silver Longbow for comparisons, please pardon the napkin math

All builds get +2 Elf Racial

20 Ranger (Non-favored)
25% Speed
+3 Ram's

12/6/2 (rogue)
+1 Fighter Enh +2 STR
+2 Dam WS
+2 Dam GWS
+1 Weap Mastery I
+1 Weap Mastery II
+1 Longbow Spec
+3 Ram's
(+4 Power Surge burst)
[+6.5 Adds Sneak Attack]

+11 (+15) Damage + 6.5 SA

18/1/1
(+30% Haste Boost)
+1 Fighter Enh +2 STR
+2 Dam WS
+2 Dam GWS
+1 Weap Mastery I
+1 Weap Mastery II
+1 Longbow Spec I
+1 Longbow Spec II
+1 threat Range
(+4 Power Surge burst)

+9 (+13) Damage
So thats +1 Damage but maybe no Rams and no Sneak Attack
But more importantly +1 threat range (x2 for critical feat) looking at silver longbow's 19-20 x3 that is 2 times out of 20 you will do x3 damage or an extra x4 per 20 or +20% extra damage

This build only gets 1 FE, and it's not enhanced
The Wizard level could also be Bard since both only need 1 Ap to allow AA
And the Ranger Level (taken for free Bow Strength) could be replaced with 1 Monk (for free toughness)
But obviously not Bard/Monk


And here is the build:
Code:
Character Plan by DDO Character Planner Version 3.5.1
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Bownanza 
Level 20 Neutral Good Elf Male
(18 Fighter \ 1 Ranger \ 1 Wizard) 
Hit Points: 314
Spell Points: 170 
BAB: 19\19\24\29\29
Fortitude: 15
Reflex: 15
Will: 7

                  Starting          Feat/Enhancement
Abilities        Base Stats          Modified Stats
(32 Point)       (Level 1)             (Level 20)
Strength             16                    18
Dexterity            18                    24
Constitution         14                    14
Intelligence         10                    10
Wisdom                8                     8
Charisma              8                     8

Level 1 (Ranger)
Feat: (Favored Enemy) Favored Enemy: Evil Outsider
Feat: (Selected) Two Weapon Fighting


Level 2 (Fighter)
Feat: (Fighter Bonus) Point Blank Shot


Level 3 (Fighter)
Feat: (Fighter Bonus) Rapid Shot
Feat: (Selected) Weapon Focus: Ranged Weapons


Level 4 (Fighter)
Ability Raise: DEX


Level 5 (Fighter)
Feat: (Fighter Bonus) Weapon Specialization: Ranged Weapons


Level 6 (Fighter)
Feat: (Selected) Toughness


Level 7 (Fighter)
Feat: (Fighter Bonus) Manyshot


Level 8 (Fighter)
Ability Raise: DEX


Level 9 (Wizard)
Feat: (Wizard Bonus) Extend Spell
Feat: (Selected) Improved Two Weapon Fighting
Spell (1): Shield
Spell (1): Jump
Spell (1): Expeditious Retreat


Level 10 (Fighter)
Feat: (Fighter Bonus) Improved Critical: Ranged Weapons


Level 11 (Fighter)


Level 12 (Fighter)
Ability Raise: DEX
Feat: (Selected) Greater Weapon Focus: Ranged Weapons
Feat: (Fighter Bonus) Improved Critical: Piercing Weapons


Level 13 (Fighter)


Level 14 (Fighter)
Feat: (Fighter Bonus) Greater Two Weapon Fighting


Level 15 (Fighter)
Feat: (Selected) Greater Weapon Specialization: Ranged Weapons


Level 16 (Fighter)
Ability Raise: DEX
Feat: (Fighter Bonus) Precise Shot


Level 17 (Fighter)


Level 18 (Fighter)
Feat: (Selected) Improved Precise Shot
Feat: (Fighter Bonus) Superior Weapon Focus: Ranged Weapons


Level 19 (Fighter)


Level 20 (Fighter)
Ability Raise: DEX
Feat: (Fighter Bonus) Power Attack
Enhancement: Fighter Attack Boost I
Enhancement: Fighter Attack Boost II
Enhancement: Fighter Attack Boost III
Enhancement: Fighter Haste Boost I
Enhancement: Fighter Haste Boost II
Enhancement: Fighter Haste Boost III
Enhancement: Fighter Haste Boost IV
Enhancement: Ranger Sprint Boost I
Enhancement: Elven Arcane Archer: Imbue Acid Arrows
Enhancement: Elven Arcane Archer: Imbue Explosive Arrows
Enhancement: Elven Arcane Archer: Imbue Force Arrows
Enhancement: Elven Arcane Archer: Imbue Force Burst Arrows
Enhancement: Elven Arcane Archer: Imbue Slaying Arrows
Enhancement: Elven Arcane Archer: Imbue Terror Arrows
Enhancement: Elven Dexterity I
Enhancement: Aerenal Elf Melee Attack I
Enhancement: Elven Ranged Attack I
Enhancement: Elven Ranged Attack II
Enhancement: Elven Ranged Damage I
Enhancement: Elven Ranged Damage II
Enhancement: Kensei Longbow Mastery I
Enhancement: Kensei Longbow Mastery II
Enhancement: Kensei Longbow Mastery III
Enhancement: Fighter Critical Accuracy I
Enhancement: Fighter Critical Accuracy II
Enhancement: Fighter Critical Accuracy III
Enhancement: Fighter Kensei I
Enhancement: Fighter Kensei II
Enhancement: Fighter Kensei III
Enhancement: Fighter Longbow Specialization I
Enhancement: Fighter Longbow Specialization II
Enhancement: Racial Toughness I
Enhancement: Racial Toughness II
Enhancement: Elven Arcane Archer: Conjure +2 Arrows
Enhancement: Elven Arcane Archer: Conjure +3 Arrows
Enhancement: Elven Arcane Archer: Conjure +4 Arrows
Enhancement: Elven Arcane Archer: Conjure +5 Arrows
Enhancement: Elven Arcane Archer I
Enhancement: Wizard Energy of the Scholar I
Enhancement: Fighter Strength I
Enhancement: Fighter Strength II
Enhancement: Fighter Toughness I
Enhancement: Fighter Toughness II