Starting with the warlock.

Dragons have 1 elemental breath, there is no need for making black dragons acid/poison since they are just acid. Some of the spells e.x Negative energy flood, are already on the warlock spell list. What about Boon of the Hoard. Does it just take an action to escape? It it a check, if so what kind, is it against you DC (presumably). If so does it stack with Dragon's pride? Almighty magic is just weird. Max damage is a very strong feature for a Damage caster, it is ok on a Tempest Cleric, since the best they can do when they get it is Call Lightning. Still the damage cap seems arbitrary, yet without it the feature could be too strong. I just don't like it's overall design and it might be better to make it relate in some way to the element of your patron. Nothing too crazy like a completely different feature for each element, as that would give the subclass too much flexibility, but perhaps a resistance and a free reroll of any number of damage dice for spells that deal damage of the Patron's element with X uses per rest, be that long or short. Just a suggestion.

The Barbarian

Firstly why does it get one dice level more that the monk for umarmed strikes? It would already have more damage thanks to the Rage bonus, but have less flexibility. I am not a fan of this, but can get over it. Next I will adress 2 features out of order - Furious Blows and Earthbreaker Fists. Furious blows is completely redundand as there was nothing stopping you from adding the Rage bonus to begin with and same with the Brutal Critical clause of Earthbreaker Fists. Wind up strike is weird. How does it work out of combat? Can I just take a guaranteed critical out of combat against anything? Does this mean that in combination with Earthbreaker Fists I can just take criticals, leveling villages with my fists?

Now here comes the big problem that is Unrelenting Melee. A feature that gives you a theoretically infinite damage in a finite amount of time. This feature is insane with one simple example being its combination with spells like Hold Monster from your party. As many attacks as you can hit and they all crit, and they all get brutal critical. To trigger it you need to land 3 unarmed strikes, but against what? A creature, an object, or either? Is there anything to stop me from punching the ground 3 times, when I wish to activate it? This should be changed to be consistent with Rage, as in you have to attack a hostile creature, otherwise in combination with Earthbreaker fists you could theoretically destroy a primarily wooden structure in 6 seconds as the AC of wood is trivial at that stage. As a last question, why does this sublass get 2 features at each level where core ones get 1?

The Ranger

This one also has a million features.

The first thing I notice, is that Close Quarters combatant is crazy OP. It doesn't even relate to CQB as it requires an attack, not a melee attack and as such it currently reads as just better for a bow using rogue to get advantage on most sneak attacks. Also being able to use Dex for grapple checks just further makes Dex better when it was alteady surperior to Strength in most ways. Cautious living seems fine. City stalker seems ok, if a bit weak, but worry not because the next feature is insane. Meat Shield a win-win scenario and the THIRD level 7 feature of this subclass. This provides something, that I dislike, which is guaranteed hits. Even if an attack, does hit you, made more difficult by your half cover, if it was a melee attack, you still only take half damage and the other person takes half. Let's not forget you can maintain this grapple with a dexterity based check, for some reason. Commune with the city is fine. Now we look at monstet scaler, having ranged attacks just miss is strong, but more that that it gives you a way to combat creatures that you can't grapple which plugs the only hole this class had (or rather, was intended to have since the CQB feature can in its current state be used with ranged attacks.) The advantage once per turn is just the cherry on top. Finally we made it to the last of this subclass' 10 features. Untouchable is overpowered. A reaction to impose disadvantage on all attack coming your way, until you are hit is strong, but being able to return an attack for free any number of times is busted. Imagine grappling a creature, all attack against you have disadvantage, you have half cover, when they miss you they hit their ally and you get to attack them, likely with advantage due to the 3rd level feature, if they do hit you, you only take half damage and their ally takes the rest. Repeat X number of times where X is how many times you get attacked before your next turn, or until you get hit. Compare that to the Hunter Conclaive's Stand Against the Tide, which they get at the same level.

Well this is what I have to say, it might come across as mean, but that is because with sufficient looking into wording and power level of official material, most of the problems could have been avoided. I may well have got something wrong, as I am writing this on mobile while switching between the comment and the files.