Unlimited (even if you don't want to be a Beholder)
While the concept is bizarre to me, some people might not want to spend their time as a Beholder... For them, the process is actually simpler.
Attain access to Finger of Death
Wander through the land, murdering hapless peasants.
A humanoid killed by this spell rises at the start of your next turn
TL;DR: A 3 levels dip is probably your best option, as it more than compensates for the abilities you will lose as a paladin considering your role in the party. Warlock 3 grants you two extra 2nd level spell slots (renewable on a short rest) that can be used to smite or heal yourself, without costing you the ability to cast 5th level spells. Celestial pact ...
There is no upper limit to the number of undead that you can control...the process is pretty simple, too.
Attain access to True Polymorph
Turn yourself into a Death Tyrant Beholder, hold the spell until it becomes permanent
Massacre vast numbers of humanoids while within your Negative Energy Cone. Target goblins or orcs if you have something ...
You start with a +4 Strength modifier at level 1
Your Constitution modifier is assumed to be +3 at level 1
ASIs go towards raising Strength to 20, the Sentinel Feat, the Savage Attacker Feat, and raising Constitution, in that order
all attacks are made with Reckless Attack, -5/+10, and receive the rage damage bonus
The enemies' AC is 16 for ...
With the "Initiative Score" initiative variant you don't need to roll dice:
With this optional rule, creature don't roll initiative at the start of combat. Instead, each creature has an initiative score, which is a passive Dexterity check: 10 + Dexterity modifier.
However, it's a Variant rule, so your DM has to explicitly allow this.
The best I could get was a 99.996% chance of winning
First we assume the NPC is not going to play optimally, but will always roll with the goal of hitting as many pins as possible.
I could not find any way to gain proficiency in improvised weapons under these restrictions, but if we could my program did not have us lose a single one of its one ...
You lose the paladin features of levels 17,18,19, and 20 depending on the dip
Functionally, this is pretty big. You not only lose some big paladin abilities, but you also delay your standard progression. I'm going to first go through what you things will be missed and will then cover my experience playing a full paladin as well as my experience in doing a ...
Aasimar Zealot Barbarian(16)/Grave Cleric (2)/Hexblade Warlock(1)/Any(1): +122 on one attack, +61 on later
Our character is an (Protector or Fallen) Aasimar level 16 Zealot Barbarian, level 2 Grave Cleric, level 1 Hexblade Warlock and has 1 level in anything, with the Great Weapon Master and Charger feats armed with a +3 heavy weapon.
Protector and Fallen ...
Potentially, becoming a mount would work
The rules on "Controlling a Mount" state:
A willing creature that is at least one size larger than you and that has an appropriate anatomy can serve as a mount, using the following rules...
Intelligent creatures, such as dragons, act independently.
You can control a mount only if it has been trained to accept ...
It depends on how long the campaign runs
You've asked about Paladin18/Warlock2 vs Paladin17/Warlock3 vs Paladin16/Warlock4, but that seems like a weird thing to focus on.
Many games don't even reach level 20. (The game gets harder to run at high levels, so games that are stable initially might not remain stable.)
Of the games that do reach level 20, most ...
An Aasimar Hexblade Warlock (1), Wild Mage Sorcerer (1), Grave Domain Cleric (2), Anything (16) with a ninth level spell slot available can get at least +272 and at most +492 damage on a single melee spell attack
This works as follows:
As pointed out in other answers the Aasimar gets the ability to transform which states:
During it, [...] once on each ...
Since your projected build requires your character to be a Dwarf for certain feats, this means that being a Fire Genasi for an easy attack power isn't possible. However, in "Wayfinder's Guide to Eberron", assuming the contents of this book are allowed at your table, there's a feat available that can grant you access to one sorcerer cantrip that uses your ...
Bonus Action Limitations of Two Weapon Fighting
Raging is a bonus action. If you want to rage, you'll be unable use two weapon fighting that turn. This can put you at a disadvantage right from the start of combat.
Great Weapon Master Bonus Actions
It's not just a critical attack that allows a great weapon master to attack with their bonus action.
Has a Medium range, while Color Spray is a 15 ft. cone. Sleep can be cast from a much safer distance, which is very important for spellcasters.
Affects creatures in a 10-ft.-radius-burst, covering 12 squares, while Color Spray covers 6 or 7 depending on placement. Depending on circumstances, either can be harder or easier to avoid hitting allies with, ...
I would suggest a Fire Genasi, with fire resistance and the Produce Flame cantrip keyed off Constitution. The rest of your barbarian build could work fine with this.
The other Genasi races also get +2 con bonus, but none of them have an attack spell.
I'll throw my own old calculations in case they will be useful for someone:
You will need:
A level 20 sorcerer able to cast Animate Dead.
A Ring of Spell Storing.
Someone other than you who can cast Animate Dead.
Animate Dead raises 2×(spell level)-5 or maintains control over 2×(spell level)-2 undead. We would have problem where we are forced ...
This depends on the level, whether you're raging or not, whether you're using berserker's frenzy, have advantage etc.
Since all combinations are a bit hard to consider, I'll make some assumptions:
Level 1 a STR score of 16 or 17 (+3) is assumed
The first ASI is used to buy Great Weapon Master, further ASIs are used to increase the STR to 20 asap
I'm going to suggest a substantially different level split:
You should take 8 levels of Warlock
You say you value 5th level Paladin spells highly, but you should not. They are weak.
Think of them this way: A 10th level Bard (with Magical Secrects) can cast any spells you could on 17th level. It is not owerpowered for him, so it must be weak for you.
Your build, character, and party
You are a low-level spellcaster—you do not have the stamina you will in a few levels. You should be aiming to decisively change the course of a battle in one spell, and then leave it to your mundane allies with mundane options to take whatever advantage you set up, and finish the encounter.
Your party is fairly magic-heavy, ...
Zealot Barbarian (20) Total static damage of +34
For this build we use the true definition of static damage; every attack turn and without single target or one a day use restrictions, such as, Hexblade's Curse which is against a sole creature or the Aasimar transformation which is once a day. We use a character with the Belt of Fire Giant Strength (sets ...
This is provided as an additional answer as it only gives it one 1 turn, and is highly situational
Goblin Rogue Assassin 17/Hexblade Warlock 1/Grave Cleric 2: +196 on a single attack
We have a +3 weapon with the heavy property and the Great Weapon Master and Charger feats.
We take a setup round (which is before initiative) using our action on Grave Domain'...
There is no limit to the melee weapon attack static damage
With a Moonblade with arbitrarily many runes, one can get many 93-94 runes which provide (emphasis mine):
When you hit a creature of a specific type (such as dragon, fiend, or undead) with the moonblade, the target takes an extra 1d6 damage of one of these types: acid, cold, fire, lightning, or ...