What are the mods you find essential?

Witches just level alchemy much faster than others. The elixirs are useful for everyone though. Even the first one you learn, the Vial of Bliss, is already very useful for filling fun and social moodlets. For witches, the ingredient (any food item) comes free (they can summon apples and often do so autonomously), but it's easy for anyone with a garden. The Invigorating Elixir comes also at level 1 and fills your energy bar to some extent. Both elixirs have even better versions at higher levels. There are 44 elixirs available to make, and quite a few of them have their uses.

The combination of witch and alchemy can be considered cheaty. You have full control of all needs, can clean and repair everything with a single spell, upgrade objects, make friends and enemies at will, etc. You don't really need anything else, except gardening.