A lot of my favorite classes peaked at 4e and then took steps backwards.
I love the Artificer, even if it isn't quite right. As soon as they got rid of magical items as a commodity (despite messing it up in 5e. Consumables still annoy me for their price compared to reusable magical items) the Artificer lost something. With spells being the be all and end all of rules bending, the artificer has to be a spellcaster, but its off somewhat.
Barberian: once it was decided that barberians were the rage guy, it got its own idenity. No real complaints and really enjoy some of the new archtypes.
Bards rock

...next class
Cleric suffer from the need to cover archtypes based on sphere of influence. This class peaked in 2e with specialty priests and the kits from fighters and priests of the realms for breadth of concepts. However 4e also did a great job breaking down the cleric role (aka divine role) into Cleric/Paladin/Avenger/invoker.
in 5e backgrounds help, but not quite right.
Druid: The single biggest mistake was not separating out shapeshifting as a dedicated class. It is a power that you could get 20 levels out of and not need spells, animal companions and other woodsy abilities Plus the insta heal makes it, while not as badly broken as it is in 3e, still has issues.
Druid needs to almost become an archtype for Cleric, as their isn't enough once you take out the shapeshifting, but I'd be okay with making it the dedicated shapeshifting class like it was in 4e.
Fighter: When the 4e fighter was made, a thousand spellcasters could be heard whining. Suddenly the fighter had a niche (heavy front line fighting) and it was really, really good at it as well as creating an agony of choice. While the fighter concept was still pretty generic, it roared.
But the idea that fighter was the simple class and absolute hatred of the warlord (by spellcasters), means they put the subclasses together on mechanics and not theme. It should have been broken down by knight, mercenary, soldier with a few tweaks to show the change in niche.
plus while I don't mind the Eldritch Knight and it plays well enough, the 4e Sword mage puts it to shame in concept and form.
and the raw hate of the Warlord class meant it was folded in to fighter with varying results (because spells are the main rules bender in the game and a class that can do similar stuff without magic feels unatural).
Monk: This one is a wierd one. I'd argue 5e did the class the best, but it doesn't feel so unecessary as misfiled. It either is a divine warrior (make a paladin archtype) or a martial artists (see fighter or swordmage, depending on magic levels).
Paladin: This class I really like and while there are nitpicks, it really talks the talk and walks the walk. I'd probably do a Warden Archtype here because it was one of my favorite classes.
Ranger: First of all, let me bring up a fact that just annoys me. Robin Hood (the archtype of all rangers) cannot be easily emulated using the 5e ranger. If i would redesigning it, I'd probably just fold it under rogue and give it a seeker (aka magical archer) or sniper archtype.
Animal companions should be a companion of the group, not a class feature. Fluffy the Tiger is a fighter substitute, not a 2nd character to play.
I've also played with the idea that the animal companion is treated like a weapon (but uses wisdom for hit and damage).
Rogue: I always found it hillarious that the rogue is the only class to successfully steal a fighter's niche and keep it from 4e to 5e: that of skirmisher fighter. It could use a little oomph, but the model and concept fits it well.
Sorcerer: I have no problem with the idea of a spellcaster with a magical bloodline, but the sorcerer really needs reworking to give it more of an effect. maybe reduce it to a half caster and give it a whole range of innate abilities, but that ship may have already sailed with the warlock.
Warlock: This class is probably the most improved of all the 5e in concept and it's the choice for simplified spellcaster.
Wizard: I really don't know what to say with wizards. They are the ultimate rules benders and even while nerfed from previous editions, they still have that feel that if you want to be the clever one, you play this because the rules of reality don't apply. The archtypes of specialist classes kinda works to curtail it, but there is honestly no real agony of choice between options.
In short, I hate to say it, but cleric, fighter, monk, ranger and sorcerer, for the reasons above