A quick run through the PHB, HotDQ Supplement, DM Basic Rules 0.1 8/12
Items
Magic Armor +3
Bracers of Defense: +2 to AC while no armor & shield
Shield: +3
Ring of Protection: +1 AC
Enemy AC Ranges
Highest ACs in MM: Practical: 22AC, Tarrasque: 25AC
Highest at low levels: 18 AC, Animated Armor
Various AC options
Barbarian Unarmored Defense (10 + 5 Dex mod + 5 Con mod + 2 Bracers of Defense)
23 AC
Barbarian 20th Unarmored Defense (10 + 5 Dex mod + 7 Con mod + 2 Bracers of Defense), Primal Champion
25 AC
Barbarian 20th Unarmored Defense (10 + 5 Dex mod + 7 Con mod + 2 Shield), Primal Champion
25 AC
Monk Unarmored Defense (10 + 5 Dex Mod + Wis Mod + 2 Bracers of Defense)
22 AC
Studded Leather (12 + 5 dex + 2 shield)
19 AC
Half Plate (15 +3 dex + 2 Shield), Medium Armor Master
20 AC
Full Plate (18 + 2 Shield)
20 AC
Sorcerer (Draconic Resilience) (13 + 5 Dex + 2 Bracers of Defense)
20 AC
Mage Armor (13 + 5 Dex + 2 Bracers of Defense)
20 AC
Bonuses to Stack
Fighter, Ranger, Paladin
Fighting Style: Defense +1
Dual Wielder
+1 AC
Ring of Protection
+1 AC
Enhancement Bonus on Shield
+3
Enhancement Bonus on Armor
+3
Max vanilla PHB/DMG Equipped AC is probably
Barbarian 20th Unarmored Defense (10 + 5 Dex mod + 7 Con mod + 5 Shield(+3) + 1 Ring of Protection), Primal Champion
28 AC
Attack Bonus soft cap
Proficiency +6
Magic Weapon +3
Ability Modifier +5 / Belt of Giant Strength +9
Fighting Style: Archery +2
Melee: +14 (15 - 35)
Archery: +16 (17 - 37)
Temporary Bonuses to AC
Class
Bardic Inspiration: Die: 1d6 - 1d12, reaction
Fighter Maneuver: Evasive Footwork: Superiority Die: 1d8 - 1d12, movement
Ranger Defensive Tactics: Multiattack Defense: +4 against subsequent attacks
Wild Magic Surge 51-52, +2
Feat
Defensive Duelist: +Proficient Bonus to AC, reaction
Combat
Half Cover: +2
3/4 Cover: +5
Spell
Barkskin: Floor of 16 AC, buff
Haste: +2 AC, buff
Shield: +5 AC, reaction
Shield of Faith: +2 AC, buff
Warding Bond: +1 AC, buff
Temporary Bonuses to Attack
Class
Bardic Inspiration: Die: 1d6 - 1d12, reaction
Cleric Channel Divinity: Guided Strike: +10
Cleric Channel Divinity: War God's Blessing: +10
Fighter Maneuver: Precision Attack: Superiority Die: 1d8 - 1d12
Paladin Channel Divity Sacred Weapon: Cha to attack
Ranger Foe Flayer: Wis to attack
Sorcerer: Wild Magic: Bend Luck: +1d4
Feat
Charger: +5
Spells
Alter Self: Natural Weapons: +1
Bless: 1d4, buff
Divine Favor: 1d4, bonus, buff
Elemental Weapon: +1, buff
Magic Weapon: +1-3, bonus, buff