I voted Ranger, Sorcerer, and Monk, but would have also voted Fighter, Druid, and Warlock if I had a chance.
My thoughts:
Ranger: the ranger needs a more firm identity, and a unique class ability. Skills becoming universal things in 3E robbed the Ranger of their uniqueness. I do want spellcasting to be a ranger's thing, and I've always thought a pet would be a way to make the ranger stand out more unique from Scout Rogues, but many disagree. Giving them a major class ability built around the idea of hunting a target, and possibly taking away their extra attack to help pay for abilities and make them more of a rogue, would be ideal to me.
Sorcerer: The sorcerer needs to feel better. Metamagic could easily be an option for all casters (and just cost higher level spell slots), while sorcerers have the advantage of spending sorcery points. But, their core features should be gaining unique abilities to make them feel like a magical creature: more dragon stuff, more wildmagic stuff, more spoopy stuff.
Monk: I think the Monk needs to be rebuilt as a spellpoint using half-caster, with options for their ki powers. This would allow monks to be built along a spectrum of more grounded to more fantastical. So many of the monks abilities are ways to spend ki, but casters get abilities in addition to their spell slots.
Fighter: the fighter needs identity. They're mechanically strong, but they really need identity. They could also benefit from having more power moved from their Subclasses to the base class, but I understand the desire for a 'simple fighter'.
Warlock: the warlock and the sorcerer share too much identity. I think the warlock needs a stronger tie to hexes and curses to make them feel like they're getting and giving something in their pact.