Zardnaar
Legend
Arguably BG3 achieves better martial-caster balance than stock 5E does, but it's a complicated road to get there. They changed a lot of stuff that benefits casters, but they also give martial characters more rules-based options, and frankly a lot of their abilities are just more powerful/reliable than they are in 5E. They also subtly nerfed a lot of spells - nerfing their AOE, their range, what exactly they do and so on - a simple example would be fireball - it has a 13ft radius instead of 20ft, and a 60ft range instead of 150ft. On top of that, the monster design is different to 5E, often with boosted HP values and extra abilities, and the battlefields tend to be very vertical, which combine with the increased options for martial characters who can shove or throw enemies off things or into each other.
Outside of combat, whilst the game makes good use of spells, a lot of the "just bypass a problem" spells are simply not in the game, or have negative consequences, so it often makes more sense to rely on skill and just buff them (the game is pretty generous in terms of letting you buff rolls with spells).
There are also a wide variety of items ranging from the decent to the wildly overpowered, many, maybe most of them not using 5E magic item rules, but instead weird and wild rules Larian have made up (for example, there are no functional, normal, charge-based wands/staves/etc. in the game, but there are ton of items which use new conditions Larian have made up like "Reverberation").
The only class which continues to be worthless except as part of multiclassing is Rogue. None of the changes they've made really help Rogues. Single-classed, their DPR is the worst in the game, and all they really have to offer is Expertise in Sleight of Hand (disarm traps, pick pockets) and Stealth (or another skill of your choice), and Thief gives a free entire bonus action at L3. So a ton of builds have Rogue 3 (Thief) in them, and basically there's never any real point going beyond that in Rogue unless you are trying to make your life harder. The best "Rogue" build is Gloomstalker 5, Fighter 4, Thief 3.
Best rogue build involves Monks. Eg Monk 6. Rogue 4, fighter 2. Ascended asterion for more fun and games.
I think there's a single classed assassin build that's viable once you figure out sneak attack mechanics. Otherwise weakest class in the game outside dips. If you do that the initiative roll is the early combat, surprise round mid combat and round 1 is late.