I voted Bard, Monk, and Sorcerer.
I've noted in
this thread how I would prefer for Bards to be Half-Casters, similar to the Artificer class. I just feel that they don't fulfill their thematic role as well as they should, and are just a huge mess class-wise that can't decide if it wants to be a Swashbuckling Sword-Dancer or Poem-Chanting Skald.
Monk is just the most thematically-restrictive class in the game, and is so tied to its source material that it feels very out of place in 5e, where other classes have largely dumped their cultural baggage and expanded from their restrictive traditional niches. The Monk has failed to evolved, even as basically every other class in D&D 5e has.
Sorcerer because of its thematic and mechanical overlap with the Warlock, the PHB and XGtE subclasses not getting automatically known spell lists, their use of Spell Slots instead of Spell Points, and their restrictive metamagics (them only having 1 unique spell on their spell list doesn't help, either).
I'm also very surprised that a lot of people are saying Artificer, especially by giving the "they don't fit into most settings" complaint. I mean, Alchemists have a thematic place in pretty much every setting. The class is pretty setting specific, but there are a ton of settings that have lore-justifications for having the class (Church of Gond and Lantan in FR, Tinker Gnomes in Dragonlance and Spelljammer, Fleshmancy in Ravenloft, Sigil and Mechanus in Planescape, Izzet and Simic Guilds in Ravnica, Purphoros followers in Theros, Gunmakers in Exandria, etc). I can see why they wouldn't necessarily fit in Dark Sun and maybe Greyhawk, but they do certainly fit into the vast majority of popular D&D worlds.