Fun > Balance.
However, you need to hit a certain pre-requisite level of balance in order to have fun, even in a single player game.
At a certain level of unbalanced, you're basically eliminating player options, because one class or path is simply far too optimal. Imagine if BG3 only had the Wizard Class (as is) + several variations of the Commoner class - you wouldn't want to play any of the commoner classes, and you'd likely get very bored of the Wizard class quickly (just look at how we're basically sick of the classes in EA already).
On the flip side, you never want to be too balanced, where choices simply become superficial (i.e. Class A and Class B both do 5 damage - except one shoots red beams and the other blue). Any system with meaningful choices will effectively have some form of optimization and hierarchy. That's why I'm usually lenient on RPGs having easy late-games - because it's a sign that builds and character choices matter (since they have to balance towards the mean).
In practice -
5E Wish Spell? Terrible. It's almost a blank cheque to do anything and is game-breaking.
5E Meteor Swarm? A bit overtuned in terms of damage (250% higher than level 8 Horrid Wilting), but thematically great, fulfills a concept, and doesn't make other classes obselete (hence deleting options) - that's good OPness.
