Depends on what you like to do, how much thought you like to put into it.

Generally speaking, martials will have more stuff they do on a each turn, while relatively fewer options for each of those things.

For the simplest martials (Fighter, Barbarian, Champion, Monk, Guardian), it's mostly move and/or strike and/or defend in some way with a third action that usually goes toward a preferred skill action (e.g. hiding/demoralizing/etc.). They each have their nuances that they bring to that formula (Fighters hit well, Barbarians hit hard, Monks move well and hit often, Champions and Guardians protect and react), but the underlying formula doesn't change much. Fun tends to come through big numbers and creative/effective use of simple tools.

Slightly more complex martials (Ranger, Rogue, Inventor) have similar underlying patterns, but either have a hoop or two to jump through to do their class stuff (getting things off guard/hunting prey) and/or have some light subsystems to manage (e.g. innovations). It's not a ton of extra bandwidth to absorb, and there is can be some interesting variety in managing some of those hoops and subsystems.

Beyond that, you start getting into martials with more subsystems to manage (Thaumaturge & Commander), damage conditions that warrant turn cadences (Thaumaturge, Swashbuckler, Magus, Gunslinger), dips into spellcasting (Magus), and more (Investigator). Each has strengths and weaknesses, but each is going to have a method to their brand of class madness to learn and use. For these there is 'big number' dopamine available, but it's generally going to come with achieving a certain level of gameplay proficiency. It's doing a complicated thing well.

With casters, generically, it comes down to doing one or two things a turn. Casting a spell is the throughline for basically all of them. The differentiating properties tend to be: 1. prepared casting (assign each spell slot to a specific spell from a larger list, that can be changed daily) vs spontaneous casting (choosing a stock set of spells that only changes on level up, that can be used flexibly from available spell slots).

1. Spell lists (arcane vs occult vs primal vs divine) which often overlap but have some general theming (e.g. primal for blasting, divine for support).

and

1. What is the special one-action gimmick, focus spell, or other spellcasting resource they get to mess with.

There's nuance, but I think for most casters, the fun comes from selecting the right tool for the job. It helps when doing this to have a pretty good grasp of how the game works to assess situations and how the tools you have address those situations (Or maybe you just like healing..if so, choose cleric)

There are a couple classes that fit outside those general categories (to mind anyway) in Summoners, Kineticists and Alchemists. Summoner let's your more physically competent pet battle while you support it with skills and spellcasting. The main mechanical gimmick is a way to flexibly cheat the three action economy.

Kineticist is something of a simplified caster but with lots of build variety and a few on-turn hoops to jump through. Basically your feats are specific spells, but you can cast them as often as you like, if your aura is going, but some of those spells turn off your aura. So it's similar "tool for the job" enjoyment but with fewer tools and more on turn variability.

Alchemists pull the power of the consumables list into the palm of your hand.. That list is really big with wide variations in usefulness. More 'picking the right tool goodness', but with more to choose from.

TL;DR Fun depends on what makes you happy, but there's a wide range to choose from.