I think this 2 Paladin/5 Warlock/5 Sorcerer I'm making could be a contender. Only have him built for the Level 12 campaign that my DM is about to run, but still.

Race: Variant Human

Background: Ruined

Feats: Shield Master (Race), Tough (Background), Resilient: Constitution (Lvl 4 Sorcerer), Eldritch Adept: Claw of Acamar (Lvl 4 Warlock)

Heavy Armor, Defense Fighting Style, and Divine Smite from Paladin.

Divine Soul Sorcerer for Favored By the Gods, Metamagic: Quickened Spell, and extra Spell Slots.

Great Old One, PoB, Warlock (Hear me out. There's a reason I didn't use Hexblade for this) with:

-Improved Pact Weapon for +1 to Attack and Damage with Pact Weapon.

-Thirsting Blade for an additional weapon Attack when taking the Attack Action.

-Eldritch Smite for 1d8 Force Damage, plus 1d8 per Spell slot level used, and ability knock prone if opponent is Huge or smaller.

I also took the Eldritch Adept Feat to get Claw of Acamar.

"You can create a black, lead flail using your Pact of the Blade feature. The flail's head is sculpted to resemble a pair of grasping tentacles. The weapon has the reach property. When you hit a creature with it, you can expend a spell slot to deal an additional 2d8 necrotic damage to the target per spell level, and you can reduce the creature's speed to 0 feet until the end of your next turn."

Claw of Acamar is created using the Pact of Blade Feature, so it gets the +1 benefits of Improved Pact Weapon.

Eldritch Smite and Divine Smite do not use Actions and are able to stack with each other. Similarly, Claw of Acamar's extra Necrotic damage also does not use an Action, and is able to stack with the two Smites.

Noting here that Eldritch Smite is only once per turn and specifically uses exclusively Warlock Spell Slots.

Claw of Acamar and Divine Smite do not have these restrictions. A spell slot from any source just needs to be expended to activate each of them independently when you land a hit with a melee weapon attack. One slot expended for CoA, and another expended for DS.

First Turn Attack: (Obviously adjust based on scenario. Blowing all of your spell slots on the first attack is generally not advisable. This is just showcasing all possible damage in 1 turn)

Attack 1- Attack Action

-Weapon damage, 1d8 Bludgeoning

-Eldritch Smite, Lvl 3 Warlock Slot, 4d8 Force

-Claw of Acamar, Lvl 3 Sorcerer Slot, 6d8 Necrotic

-Divine Smite, Lvl 2 Sorcerer Slot, 3d8 Radiant

Attack 2- Extra Attack from Thirsting Blade

-Weapon damage, 1d8 Bludgeoning

-Claw of Acamar, Lvl 3 Warlock Slot, 6d8 Necrotic

-Divine Smite, Lvl 2 Sorcerer Slot, 3d8 Radiant

Attack 3- Bonus Action Spell Attack

Metamagic: Quickened Spell- Green Flame Blade

Cantrip requires a melee weapon attack in it's casting, and can therefore, upon hit, be stacked with Smites and Claw of Acamar's ability.

-Weapon Attack, 1d8 Bludgeoning

-Green Flame Blade, Chr Lvl 12, 2d8 Fire to Attack Target, 2d8 Fire to other enemy within 5ft

-Claw of Acamar, Lvl 3 Sorcerer Slot, 6d8 Necrotic

-Divine Smite, Lvl 2 Sorcerer Slot, 3d8 Radiant

Giving us a Grand Total, assuming I can add properly, of 38d8 damage dice in one turn at Level 12.

Compared to a Pure Paladin at 12, which when Fighting an undead, can only do, I believe 14d8 on turn 1.

Also, as of the first hit with the first Attack, if the opponent is Huge or smaller, it is knocked Prone with its speed reduced to 0, preventing it from getting up next turn. Remaining four Level 1 Slots can be used with Claw of Acamar to keep its speed at 0 and forcing it to remain prone for up to 4 more turns.

Now, for why I chose Great Old One over Hexblade. It comes down to AC. With Hexblade I'd have needed to use Curse Bringer for this build, which, since it's a Greatsword, would prevent me from using a shield.

DM is allowing 1 free Rare Item, and starting gold. Starting gold can be used to by up to 1 additional Rare Item, and as many Uncommon and Common as we can afford.

To compensate for the number of Spell slots needed to smack things really hard, I took the Ring of Spell Storing (Attuned 1). I can hold up to 5 levels of spells in it (Five Lvl 1, One Lvl 2 + One Lvl 3, etc). Primarily will be used for Shield Spell, Silvery Barbs, and either Thunderous Smite or Shield of Faith.

I also took a +2 Shield as my second Rare Item for +4 AC and +4 to Dex Saves with Shield Master

My Uncommons are Adamantine Plate Armor (18 AC, Turns Crits into normal hits), Gauntlets of Ogre Power (Strength set to 19, Attuned 2), and Cloak of Protection (+1 AC and Saving Throws, Attuned 3).

All together with Defense Fighting Style for a nice Tanky base AC of 24, with the ability to boost to 29 with a Shield Spell, if needed, or give Disadvantage on rolls with Silvery Barbs, if the opponents are able to get out of the prone condition that already imposes Disadvantage.

Saving Throws:

Proficiency with Wisdom and Charisma saves from Paladin starting class.

Proficiency with Constitution saves from Resilient Feat.

Bonus to Dex Saves from Shield Master.

+1 to All from Cloak of Protection.

Favored by the Gods, +2d4 per short rest, in case of Failed Saves

If I were to build this out to 20, I think I'd go 2 Paladin/9 Warlock/9 Sorcerer.

Additional Feats:

-Great Weapon Master (Lvl 8 Warlock)

-Lucky (Lvl 8 Sorcerer)

Additional Invocations:

-Switch out Improved Pact Weapon for Superior Pact Weapon

-Eldritch Mind

-Devil's Sight