Bards in general.
All of these without even considering a subclass. I'm not stating that it's overpowered, I'm just saying that their MASSIVE range of versatility steps on the toes of every other class. Clerics and Paladins focus more on healing than any other class, Rogues focus on being skill monkeys more than any other class, Wizards focus more on variety of spellcasting than any other class - but ONE bard can cover the lack of all of these classes with very little hassle, and still handle being a fighter on the frontline.
A bard at a table can easily make any other player feel like their own class is ineffective, and it's incredibly disheartening to see as a DM when players want to pitch in for a plan, but the bard can cover almost all (or ACTUALLY all) of it without anyone else's need to participate. And, worse yet, it's not a problem caused by the player that's playing the bard, as they're playing the bard as intended by the rules.
As recently as yesterday, as u/magikot pointed out here (and was upvoted by a huge margin in compared to the next highest comment chain), bards are pretty much superior to any other class, and I feel that - if WotC was to reflavor the class and release it tomorrow as a brand new class - it would be viewed as broken because of the amount of innate options.