The 12 Days of Critmas: The Paladin
Hurt people hurt people.
When building paladins for single turn kills, the approach they’ll take is to hit the enemy twice with GWM and a greataxe using divine smite on both of these (one at 1st level, the other at 2nd level so they can upcast inflict wounds). Then, with metamagic adept and shadow touched, they'll quicken second level inflict wounds for 40 (4d10) damage. To get three feats, these paladins will be variant humans. They’ll also have magic weapon up and running throughout this. There aren’t that many decision points in making a paladin for pure damage: none of the available fighting styles increase damage, and the only other real choice is subclass. In terms of subclass, they all get access to new spells, but none of these can beat inflict wounds for damage. Devotion paladins get to add their charisma to damage rolls, but so does the oathbreaker, who also gets inflict wounds for free and can pick up martial adept to improve their damage. As they have access to this feat, they’re also preferable to vengeance paladins, who can also add 1d6 damage with hunter's mark, but don’t get to add charisma to their damage rolls. None of the other subclasses offer meaningful damage improvements.
This leaves us with a variant human oathbreaker paladin, with GWM, metamagic adept, and martial adept. They deal 142 (1d12 +3 +1 +2 +10 +3d8 +1d6 +1d12 +3 +1 +2 +10 +2d8 +4d10) damage on their turn, against a non undead or fiend. If they are fighting evil (maybe they re-break their oath), this increases to 158 damage with the bonus d8s to divine smite. And, with crits on everything, they do 252 damage to most creatures and 284 to their allies.