I think they’re all fun to play, but voted witch since I find them to be the most versatile.
Riding brooms is a fun way to travel (witch children’s brooms have training wheels) or perform stunts with at the broom arena.
Witches tend to want to “play with magic” fairly often, so it’s a simple way to fulfill some wishes while leveling up their magic skill. As they advance in their witch abilities, they can learn a lot of useful charms and spells, from upgrading appliances, to cleaning up the lot and repairing broken objects, to curing negative moodlets. They can light fire pits or fireplaces with their fire blast spell, and put any fires out with ice blast. Cursing other Sims or turning them into toads can be fun too.
Conjuring apples and then using the conversion ritual to transmute them into other items is good for making money early on or for using as alchemy ingredients. Witches level up in the alchemy skill faster than other Sims, and elixir creation is very in-depth. Throwing potions at other Sims is also entertaining!
You can also use the Philosopher’s Stone (LTR) to transmute cheap or otherwise useless items like old newspapers into gold ingots, which you can further magically convert for alchemy ingredients. Sometimes the transmutation can fail though, which could kill your Sim if you don’t have a death flower on hand. The Philosopher’s Stone can also be used to bind ghosts.
When SN and Seasons are both installed, the Weather Stone object will appear on a community lot in your town. Each SN occult type has a special weather affect that they can summon, which is more likely to be successful the higher their magic skill (if it fails, you may get struck with a lightning bolt). Witches can cast Bewitching Rain that makes a green rain fall. It boosts the mood of Sims outside and makes them dance, lol.
Also, if you have any of the Magic premium content from the Store, those will enhance the theme and fill a witch’s magic room nicely. IIRC, using the Book of Spells or the Voodoo doll increases the spellcasting skill, and using the Cauldron increases the alchemy skill while conjuring various ingredients needed for crafting elixirs.
