Getting ready for the EE. Planning a Demon Witch Enchanter/Necromancer playthrough.
The witch spellbook takes some getting used to since it is a hybrid spell book, supplemented with patron spells. Overall it is not a popular class, judging from the lack of "builds" online. So I am having a little look.
Overall either Enchantment Specialism or Necromancy Specialism is the way to go.
Looking at Patrons (grant extra spells)
Agility: Expeditious Retreat ,Cat's Grace, Haste, Freedom of Movement, Polymorph, Cat's Grace, Mass Polymorph, Greater Polymorph, Greater Polymorph, Greater
Unless you desperately need Haste and don't have Nenio or Woljif that's a hard no.
Ancestors: bless, aid, prayer, shield of dawn, burst of glory, greater heroism, Bestow Grace of Champion ,Angelic Aspect Greater ,weird
Not the worst spells in the world but a strange mix. Aid, Prayer, Greater Heroism are all good spells but outshined by some other patrons.
Deception: Vanish Invisibility Displacement Confusion Phantasmal Web Serenity Invisibility, Mass Angelic Aspect, Greater Weird
A mix of enchantment and illusion spells. Weird and Phantasmal Web are nice and probably the best bet for rounding out an illusion witch spellbook.
Devotion: Divine Favor Invisibility Magical Vestment Confusion Flame Strike Bull's Strength, Mass Bestow Grace of the Champion Holy Aura Heal, Mass
An ok mix: Divine Favour to help rays hit, Heal, Mass; still lacking imo
Elements: Shocking Grasp, Acid Arrow, Fireball, Dragon's Breath, Flame Strike, Acid Fog, Caustic Eruption, Polar Ray, Icy Prison, Mass
A very nice mix of offensive spells if you want an Evocation Witch
Endurance: Stunning Barrier Bear's Endurance Protection From Energy Thorn Body Spell Resistance Bear's Endurance, Mass Restoration, Greater Polar Ray Seamantle
Spell Resistance is useful through the game - watching regular enemies spells bounce off you is nice. Unfortunately not much else to offer beyond the high level addition of Polar Ray and Seamantle
Healing: Remove Fear, Restoration, Lesser, Remove Disease Restoration, Cleanse, Pillar of Life, Restoration, Greater, Cure Critical Wounds, Mass; Heal, Mass
Some nice additions for a healing focused Witch (c.f. Hex Channeler).
Insanity: Color Spray Hideous Laughter Hold Person Confusion Mind Fog Serenity Insanity Cure Critical Wounds, Mass Overwhelming Presence
All sorts of fantastic Enchantment spells which will see use throughout the game (immunes excluded). A very nice selection which would be more useful for a Spontaneous Witch. However Hideous Laughter is a very nice add for any witch.
Mercy: Shocking Grasp Eagle's Splendor Remove Curse Restoration Cleanse Inspiring Recovery Resurrection Euphoric Tranquility Heal, Mass
A weaker equivalent to Healing patron.
Shadow: Vanish Blur Displacement Shadow Conjuration Shadow Evocation Phantasmal Putrefaction Shadow Conjuration, Greater Shadow Evocation, Greater Shades
The obvious choice if you want to do a Shadow based Witch. I am not sure if the Shadow Evocation spell DCs are still janky or not.
Strength: Divine Favor Bull's Strength Heroism Divine Power Righteous Might Bull's Strength, Mass Legendary Proportions Frightful Aspect Heroic Invocation
Fantastic spell selection. Especially useful for a ray-slinger with Divine Power. Almost everything here is useful.
Transformation: Longstrider,Bear's Endurance Beast Shape I Beast Shape II Beast Shape III Dragonkind I Dragonkind II Dragonkind III Shapechange
If you want to be a dragon... if not...
Winter: Snowball Resist Fire Protection from Fire Ice Storm Icy Prison Cold Ice Strike Ice Body Polar Ray Polar Midnight
Honestly painful. I hate this so much.
I think on the face of it, depending on your main focus, Strength, Insanity, Deception, Shadow, Healing, or Elements are the strongest, based on your specific desires for the witch.