Nobody knows this yet, unless you have perfect knowledge of the game’s items. A lot of them are so powerful they can easily be build defining and can swing the balance one way or the other.
It’s also worth noting that with multiclassing some classes can be a great dip, some can be a great main or pure class and some can be both.
Main Classes
S Tier
Sorcerer – Great aoe and single target damage with wet + cold/lightning spells, great nova with Haste + Quicken, CC with Heighten, Good survivability with Shield + multiclassing or certain items. Tons of good multiclass options, but good as a pure build too.
A Tier
Monk – Specifically strength build with Way of the Open Palm with Tavern Brawler has some of the best dpr in the game levels 4-10, and even 11+ it still keeps up with the best, while having +5 to hit from Tavern Brawler instead of -5 of Sharpshooter/GWM. Benefits from multiclassing a lot.
Druid – Moon druids are one of the few classes getting 3 attacks per action, even 1 level sooner than fighters. It is a bit harder to stack damage on them, but it’s still respectable and they have an insane pool of hp between shapes and summons. Summons getting changed to non concentration spells means you can run a small army of summons + concentrate on a spell + wildshape, which is very strong. Mostly seems good as pure class.
Wizard – Rated lower than Sorc cause no Twin & Quicken, lowering power of Concentration spells and nova potential as well as less good multiclass options. Still has access to the same good spells and more; and some subclasses are pretty insane e.g. the reworked Abjuration Wizard is basically unkillable if built and played right, while enemies die from just touching him due to retaliation damage. Evokers are nice too though come onto their own a bit late, Diviners and Enchanters great as ever. Staff of Cherished Necromancy makes Necromancer a potential S tier.
Bard – Great utility & support class, but College of Swords is very capable in combat too with how Slashing Flourish is implemented right now. Lore is more of a support option but can destroy bosses saves which is great for neutering them with things like Polymorph or Hold Monster.
Cleric – Another good utility & support class but can be good at blasting too depending on domain choice. In many ways, a wizard with much better survivability, though lacking some of the more powerful spells and spellcasting passives wizard subclasses get. Headband of Intellect and 1 wizard level dip makes you one of the most versatile spellcasters.
B Tier
Fighter – Generally a pretty strong class, bonus feat allows to get some combos like GWM and Polearm master easier, action surge is decent though it pales compared to Haste (then again it can be stacked), Battlemaster stuff is pretty good though limited. Fighters have a huge power spike at level 11 with 3 attacks per action, which makes them some of the best damage dealers. However there are currently stronger builds with similar amount of attacks/dpr that offer more utility and play better levels 5-10 (e.g. strength Monk 6/Thief 4/Fighter 2 or Paladin 7/Pact of the Blade Warlock 5 or Swords Bard 6/Pact of the Blade Warlock 5/Cleric 1) that’s why I’m putting them in B tier, but it’s a high B.
Paladin – I’d maybe even put them in C if not for the Pact of the Blade multiclass with 3 attacks per action right now. Auras are nice but very small radius and make you vulnerable to AoEs if you bunch up to use them. Smiting is good damage but very limited and in general the class burns resources super fast while a Monk can output a similar dpr burning nothing. However Pact of the Blade multiclass can push this potentially even to A tier with up to 3 attacks/round on level 10, single attribute dependency and extra smites on short rest. However as a whole i can’t give it more than B.
Warlock – Similarly to paladins i don’t think Warlocks are very good on their own, however they can be a part of some A and even S tier builds as a 2-5 level dip. 2 levels get you Eldritch Blast with Agonizing Blast for ranged dpr (item dependant for an actually good damage output but can get crazy with the right setup). Or a 5 level dip for Pact of the Blade with an extra attack on Paladins, Bards or even Rangers. Again, on their own can’t give more than a B though.
C Tier
Barbarian – Very strong at early levels, pretty much S tier on levels 1-4 but starts to fall off rapidly after that. Generally changes to Haste screw this class pretty bad, and multiclassing with Warlock to get 3 attacks per round doesn’t feel great as you want more Barb to make your Rage better.
Ranger – Tbh my least used class so I might be missing a lot, but apart from round 1 meme nova builds with Gloomstalker it feels like a worse version of Paladin with less synergistic multiclassing and less utility.
Rogue – Skill monkey class, however Bards do that and more, Thief 3-4 and Assassin 3-4 can be A tier dips in some builds though.
Secondary Classes / Dips
S Tier
Wizard 1-2 dip for scrolls, Warlock 2 dip for EB, Thief 3-4 for extra bonus action, Assassin 3-4 for crits and double surprise turn, Pact of the Blade Warlock 5 for extra attack and CHA to attack and damage
A Tier
Cleric 1-2 for proficiencies and caster level, channel divinity, Fighter 1-2 for CON saves + proficiencies, action surge
I have tried Bard, Fighter, Monk and wizard.
Fighter is as expected the best at combat but do not contribute much elsewhere. But combat is the critical part of the game.
Bards Collage of swords compares well with a fighter battle master. Its not as good at combat, but not far off. Where the bard gains is everywhere else. If one believes that parts of the game other than combat are important then the bard shines. Anyway its my preferred class.
Not tried Paladin yet. But they probably work similarly to Bards; almost as good as fighters at hitting people with bits of metal, but with lots of other stuff that fighters lack.
Wizards – well when I played as a fighter or bard I was always taking Gale with me as some spells are so useful. Like magic missiles to make sure you get rid of those one hit point enemies. Anyway playing a wizard means you can leave Gale behind and take both Lae’zel and Karlach with you.
Monk. I only played up to level 5, maybe it gets better. But I just preferred the standard fighter.
Bard,
Fighter
Wizard, Monk