Least favourite occult

out of only those 3?
then I must say Spellcasters

the thing is other occults can't be spellcasters
so I can't say make sea witch that is mermaid and spellcaster simultaneously or werewolf that is spellcaster too or whatever

and kids cannot do magic so rip any fantasy movie ever basically

and there really isn't all that much stuff to do just being a spellcaster

like no show magic, no magical pets, no magic mirrors, no fortune telling, no strange rituals, flying on broom is just teleportation, familiars are just drones and the spells are mostly just cheats but fancy... its just kinda meh

out of all occult though id say plantsims cause its not even actual occult state in ts4

Out of just those three, werewolves.

They are all better than I expected honestly, more fleshed out than in other games, and that was the only metric I was going on. I don't dislike any of them, I just prefer playing vampires and regular Sims and always have. I've played Spellcasters a few times (in one family) and werewolves only once.

If I had to pick one you didn't list, like Simmingal, it would be Plantsims.

There are two different ways to answer this.

On a gameplay level, even though they're the occult I actually play the most, I'd have to agree that Spellcasters are the most bland. There's a lot they can't do (though I personally don't mind the lack of crossover too much. Sam Haight still haunts my TTRPG nightmares, and I never actually used him in my games) I like to play them because I like the spell cheats, and the other two occults have either hunger or rage that has to be managed, while Spellcasters can just be normal people with magic. But I do acknowledge that they are the weakest - gameplay wise - of the three.

On a personal level, I've never really liked playing vampires. Not Vampires in the Sims, just playing vampires in general. Villain Protagonists aren't stories I'm interested in exploring, and I was over Angsty/Friendly Neighborhood Vampires well before Twilight was ever put to page.

Spellcasters are the worst. They are not occult, but rather secret society with access to secret knowledge, all their powers come from studying the spell books.

Out of those 3, werewolves for me. I don't dislike them, but they're my least favourite to play with. I don't like that they can't even hunt when rampaging, and they just don't seem to behave as you would expect a werewolf to. Still, they do have to be family friendly with them I guess. :D

Vampires and Werewolves. I have several vampires and werewolves in my game as background characters and friends but I don't play them at all. Spellcasters are my favorite.

Just like @DaWaterRat said there are two ways to answer this question: least favorite gameplay or least favorite theme. For me the answer is different for these two ways to consider occults.

Gameplay: of the big three, spellcasters are obviously the weakest. They are imbalanced and overpowered. There are hardly any drawbacks for being a spellcaster. Vampires are hard because of the deadly sunlight and the thirst need. Werewolves can be real challenging because of the temperaments. But spellcasters? Potions and spells rarely fail. And if they do it is easy to get rid of the curse by a simple spell, magical potion or a cure bought at the magic shops. Some of the potions and spells are overpowered and allow an advantage that is just downright cheating. My solution to this problem is self imposed restrictions on my sims and mods. My sims are not allowed to get all perks. They can only unlock one perk for spells that never fail, in other wordt they have to specialize in one branch of magic and those spells never fail. But the other two branches can fail. Of the 3 linear colomns of perks, they can only fully develop one to the top, the other two only the lower perks. I also limit the use of spells like copypasto. My spellcasters never use it to earn money, copy potions or copy harvestables. They basically only use it for those potions ingredients you can't grow like crystals. I also have a mod for the potion of plentyful needs so it doesn't fill all needs and requires more ingredient. With these rules and mods, they I enjoy playing them.
Another huge letdown for spellcasters is the lack of attention for children. It really hurts that children in the sims can't see, hear, touch or smell magic and can't be touched by magic at all. A child practice wand or an option for spellcasters to scruberoo their offspring would have made a difference.
Realm of magic unfortunatly has other missed opportunities as well. No lot traits for magical babies (like the vampire dark ley line) , no lot trait to attract spellcasters or allow magical behavious and therefor no way to create any magical lot outside of the ever dark magical realm, brooms as teleport instead of flying, familiars that are only visual effects and can't be touched in any way, not a single cas item for toddlers and children,...

Thematically: vampires. This is mainly a personal preference. I dislike the need to feed on other sims. It is great to have an alternative in plasma packs but the nature of the thirst urge itself remains and I don't like it. Despite the fact that I don't like them I have to admit that vampires are excellently implemented in terms of gameplay.

What about the third main occult werewolves? They are excellent in my opinion. I greatly enjoy them even if they rank in the middle in both gameplay and theme for me. I like how they look (if only we had a female bodyframe available) and the gameplay is balanced and challenging. Werewolves have great perks to unlock but it comes at a price. I also really enjoy the face that toddlers and children can get werewolf related moodlets. It's a small details but just enough to make them feel different from regular children.

In general I play a lot of spellcasters and werewolves, occasionally a mermaid or alien. Vampires are very few in my game and with all the other occults options, I rarely touch them. Should vampires have been released as third in depth occult, I might have skipped them. But because we hardly had any occults when vampires were released I got the pack.

I agree with @Simmingal when it comes to all occults, not just the 3 main ones. In that case plantsims are the biggest letdown. They were downgraded from a full lifestate in sims 2 and 3 to nothing but temporary state in sims 4. To me it almost feels as if plantsims in sims 4 are a desease rather than a lifestate: No powers, no way to make plantsims permanent. And the worst of all: as soon as you switch households, the plantsim moodlet is not shown, only your active household can have a plantsim. I hope they get and upgrade/revamp at some time. It has happened before (zombies in sims 2 university returned in sims 2 freetime, vampires in sims 3 late night and supernatural and robots in sims 3 ambitions and into the future) so there is still a sliver of hope for plantsims in sims 4.

Ghost sims, they have no proper ghost abilities plus they always love to break things.
Also what’s up with them needing to eat, sleep, use the toilet & bathe. Seriously, they’re Ghosts for goodness sake, oi!

to be honest i dislike all ocult in the game to the point that i do not play them but on occasion i play alien sims family but not vampires witches wolves and mermaid

Yeah, plant sims suck in TS4. It's not a permanent state, and they didn't even get proper plant leaves for hair like back in TS2, just the green version of one of the existing hairstyles. (Also, I have a bug that makes plant sims' hair change all the time.) No plant vines on their skin either if I remember correctly, just the regular green skin that some aliens have. The prize for the most minimal effort ever put into an occult goes to the creators of the TS4 plant sims.

Of the three main occults mentioned in the first post, werewolves are my least favourite. Too high-maintenance.