**Dual-wielding Monk**

A simple way is to be a Monk with the Dual Wielder feat, 20 Dex, and 20 Wis.  This gives you a 20 AC from Unarmoured Defence, with +1 from the Dual Wielder feat for a total of 21.

This can be increased by 2 in most combat rounds if you're a Kensai. If you use a kensai weapon and a monk weapon in your hands to count as Dual Wielding, you can make unarmed attacks by kicking your enemies. Any round you kick using your Attack action gives you +2 to AC from the Kensai Agile Parry feature, for a total of 23.

**Defensive Style**

A Fighter, Ranger, or Paladin can take the Defensive Style feature, which gives them +1 AC when wearing armour.  This gives them a maximum of 21 (18 from plate, +2 shield, +1 style).