It depends what you don't like in the game. However, going class by class, mostly from memory:
Barbarian: Berserker's core feature ties into an exhaustion mechanic nothing else uses and the mechanic just doesn't fit the game overall (might be great rules for some variant version of 5e, but a poor fit for how most people play it). Nearly all the Babarian subclasses are heavily magic based, which kind of goes against the core theme of the class from many people's perspectives.
Bard: All subclasses should have a means of spending their inspiration directly rather than giving it to someone else. I know this goes against some people's core idea of what a Bard should be because it's been primarily a party buffing class in other editions, but it's just two common to have fellow players who just never use the inspiration dice you give out, and they are a major resource for the Bard.
Cleric: Not a big Cleric player so no strong opinions. It always struck me as really dumb that the Forge Cleric didn't have martial weapons proficiency. The Trickery Cleric's "can only buff someone else's stealth" ability should be able to buff themself, because that's how nearly all abilities work in 5e and this cleric is likely weak in. the stealth department anyway. I don't necessarily care that the Twilight Cleric's aura is OP, but it should grant a set amount of temp HP rather than demand a bunch of tedious rolling.
Druid: Moon Druid is simply overpowered vs. all competitors.
Fighter: Arcane Archer needs more uses of its core ability, Purple Dragon Knight is a hopeless trashfire, a few more thematically non-magical subclasses would be good given that this is one of the principle non-magic classes. Post Tasha's version of Bladesinger the Eldritch Knight really needed a comparable replace attack with a cantrip ability.
Monk: Four elements monk simply should not be casting spells with Ki. The Ki budget of Monks is a mess already, and this mega Ki hungry subclass is a mess. Otherwise no strong opinions on subclasses.
Paladin: I feel like the Paladin subclasses are all pretty strong eventually but some have much stronger level 3 abilities than others. Generally a lot of the Channel Divinity abilities are underwhelming (I think Channel Divinity in general narrowed design space for Cleric and Paladin subclasses too much and led to a lot of samey abilities, and a lot nobody bothers to use).
Ranger: Beastmaster is a mess, and powers its mess with a cumbersome rules set. Hunter is strong, but along with Beastmaster doesn't really fit the standardized mold of later subclasses. Gloomstalker is head and shoulders the best. All the post-PHB subclasses double down on making Ranger's abiliies revolve around magic, which many of us don't care for (at least not all the time).
Rogue: Assassin's exellent Assassinate ability is two not directly related abilities confusingly presented as one, and most the rest of their abilities are crap. Inquisitive's Eye for Detail abiliy to make perception and investigation checks as bonus actions is basically useless, because nobody goes around making those sorts of checks in combat, except sometimes as a DM prompted free action. The Scout Survivalist ability is one of many skill granting abiliies that incentivizes going from zero to prodigy, because it is clearly optimal to not waste skill proficiencies until the ability grants expertise in Nature and Survival (but doesn't call it that).
Sorcerer: The fundamental issue with the class was being kept from having enough spells. This was finally solved with the Tasha's subclasses which gave a bunch of free spells. The other subclasses all needed this treatment to be worth playing (and let the Storm Sorcerer have Call Lightning!).
Warlock: Hexblade is the most ridiculously overpowered one level dip in the game for any character with high charisma. The decision to just give Warlock subclasses additional spells they can learn rather than a bunch of free known or prepared spells like subclasses in other classes was confusing and an unnecessary limitation.
Wizard: Transmuter is the weakest, War Wizard is an amazing level 2 and then a bunch of sucky abiliies, making it much more useful as a 2 level dip for an Eldritch Knight or something then as an actual subclass to play for a proper Wizard. Necromancer is the most disappointing because you have to wait to level 5 to do any real Necromancy, and then as an insult you get Animae Dead for free if you wait yet anoher level. I should be something you can cast in some limited way from he point you gain the subclass.
Overall, as the edition developed, and more pronounced still in 2024 D&D, the subclasses involved less unique and evocative abilities, and more playing it safe formulaic abilities similar to those of some other subclass. The designers moved more and more towards coralling any really distictive ability idea into being a magic spell and just giving out spells like candy to most subclasses.
