Bracers of Defense essentially give you the same as a shield, but without armor. That's most useful for people using the base AC and comes close High Base category. It's also quite useful for a Monk/Barbarian, giving them an absolute max of 22.

Robe of the Archmagi with it's 15+dex seems quite nice, but it's equal to Mage armor and Bracers of Defense.

Fighting style: Defense gives you +1 while wearing armor.

Ignored effects: Wild Magic's Spectral Shield +2, Multi attack defense +4 for remaining attacks, Cover +2/+5, Haste +2 AC 1m, Shield +5 1 round, Shield of Faith +2 10m

.	Formula	Reasonable Max	With Shield	Note
Barbarian	10 + Dex + (con or wis)	19	21	Assuming reasonable max scores of of 20/18
Base	10 + dex	15	17	.
Heavy Armor	AC of armor	18	20	no dex
High Base	13 + Dex	18	20	Dragonborn, Mage armor
Light Armor	12 + dex	17	19
Medium Armor	15 + dex (max 2)	17	19
Monk	10 + dex + wis	19	-	Assuming reasonable max scores of of 20/18
Robe of the Magi	15 + Dex	20	22	when not wearing armor
bladesinger	12 + dex + int	21	22	light armor, twice for 1m / short rest