The list can be long, I will try my best. These are the run defining spell immunities, in order from the common to the rare:
1. Harm / Mass harm. Found on variety of lowbie gear as well as LL sets and UR. The usefulness is somewhat diminished, because all it takes to counter Harm is neg potion.
2. Implosion. This is one of the oldest instakills in the mod, with corresponding amount of immunity gear. Some set items are: Ssithrak Skirmisher Scale, Tragidore robes, Penumbrium Plate, Cruel Relief, Asmo shield, Asmo belt, Crown of Corruption, Integrity Ward and many URs and BURs.
3. Breach and Mord. Do I need to elaborate?
Breach is commonly found on rings, set and random boots, cloaks or Thaumaturgic Nullifier (aboleth shield). Mord is commonly found on rings or Breach in the Weave (limbo 2 boots).
4. Bigby 6-9. Typically found together, although Bigby 5 is rarely included. For instance Ectoplasmic Weave or Asmo cloak, Asmo shield and many URs and BURs.
5. Flesh to Stone. Found for instance on Asmo plate, Sandals of Perdurance, Integrity Ward, GLory of Toril and some randoms (e.g. Splint of Elril).
6. Disintegrate. Found on Cruel Relief, Penumbrium Plate, Integrity Ward.
7. Weird. Found on Empyrean Lenses, Logos, Horrified Silence.
8. PWK. Found for instance on Mortal Failings, Essence of the Last Word, Logos or Lifebane Crown. If you are absolutely desperate, you can buy Lathar's Last Belt in item shop.
9. Dispel. Found on Eldritch Circle, Chaos Ward and a lowbie cleric helm that can be equipped with sufficient UMD.
Some niche spells that are not strictly required for runs, but can be handy:
1. Earthquake. Asmo plate and Gyroscope.
2. Meteor Swarm. Position of Power and some URs and BURs. Note that Meteor Swarm (arcane) is a different spell than Bombardment (druid). Bombardment has no immunity as far as I know, and can be equally as deadly.
3. Finger of Death. Similar case as Harm, can be countered by neg potion or shadow potion. For instance Asmo cloak.
4. Drown. Found on basic water ring, Penumbrium Plate and more.
5. Spell Immunity: Silence. Different from Immunity: Silence. Currently useful in Ely and Abo if the bard is busy/slacking. Found Logos for instance, or can be easily added by augmenter.
6. Great Thunderclap. Mostly needed for Limbo (eolians). Found on Talisman of Pazunia, some URs (Plate of Thokul Zeben), can be added with Quixart's aug.
7. ILMS + IGMS. Mainly on lowbie gear, because IGMS mostly appear in lowbie areas.
8. Word of Faith.
Special case immunities:
1. Items / potions with Shadow Shield. Blanket protection from all necromancy, including some really nasty ones - Wail, FoD, Destructs etc. Can be found virtually on all caster staves and accessories, others can drink potions.
2. Spell immunity to level X. Lvl 6 is the highest that can (as far as I know) spawn on items. Asmo amulet has unlimited casts of (dispellable) CL 60 Globe of Invulnerability giving immunity to 7 and below.
3. There are series of set shields from LL runs that each give specific immunity to wide array of elemental damage spells: Redscale Bastion (fire), Abhorrent Cold (cold), Chitinous Curtain (acid), Star of the Deep (sonic) and pretty sure there is one for elec (from Rona?), but can't seem to find it right now
4. Set of rings from Abyss, each gives blanket immunity to a whole spell school.
5. Spell Immunity: Entangle + Spell Immunity: Contagion. I suspect these protect from scripted abilities, rather than the actual spells.
Complete outliers that get used rarely (if ever), but are present on gear nonetheless:
1. Bestow Curse (Humbaba's Cruel Heart)
2. Mind Fog (Many as One)
3. ED + Enervate (Bonds of Decay)
4. Storm of Vengeance
5. Hammer of Gods
6. Creeping Doom (Thoraxian Shell)
and many more