Spellcasting is the most powerful class feature in the game. However, some versions of this feature are way better than others. The most notable variants are those of cleric, druid, and shaman, but witch also gets a special mention. Paladin, antipaladin, and ranger are also of interest, but not quite as structurally relevant.
Cleric, druid, and shaman all prepare from their entire spell list every single day. This means they can very easily prepare extremely situational spells only when those situations come up, without having to exert effort into learning them. Paladins, antipaladins, warpriests, and rangers are the same, but are less of a consideration – their spell lists are much worse, and they are not 9/9 casters.
Witch spellcasting is not quite as convenient, but it’s still a lot better than anything available to the other prepared arcane casters. Any witch can teach any other witch all of the spells she knows for no material cost – the only expenditure is time, and it requires the ability to make a DC 15 + spell level Spellcraft check. It seems reasonable to assume that most witches living in societies that have a decent amount of witches should have familiars that know most spells from their list, and a newcomer with even a single unknown spell should be quickly able to trade it for a large fraction of the other ones.
This is a list of the most useful situational spells in the game, ones which we expect to have reasonably large setting implications if utilised properly.

For solving food.

Populations generally require food to survive, and it’s good to know how many people casters can sustain. This is a collection of the available options. Some of them are bad, but still included – it’s easier to know that something is bad if you know it exists and what the same-cost options are.
A person-day of food feeds one person for one day.

Create Water.
Cleric/oracle 0, druid 0, inquisitor 0, paladin 1, shaman 0.
Effectively unlimited water.

Purify Food and Drink.
Cleric/oracle 0, druid 0, shaman 0.
Makes all sources of food available freely usable – you can eat corpses during a plague and drink from poisoned wells, and it scales enough large populations can be fine so long as a single cleric is around.

Goodberry.
Druid 1, shaman 1.
Five person-days of food if you have access to freshly-picked berries. Spoiled ones are probably fine too, druids and shamans can cast Purify.

Dream Feast.
Cleric/oracle 1, druid 1, ranger 1, shaman 1.
1 person-day. Notable for being a way for casters to trade off one first-level spell slot for never having to eat again. Better than Abstemiousness in the vast majority of situations.

Preserve.
Adept 1, cleric 1, medium 1, occultist 1, ranger 1, shaman 1, sorcerer/wizard 1, spiritualist 1, witch 1.
1 person-day of food per level doesn’t spoil for a week. Just get a cleric to Purify it later, that’s probably better.

Abstemiousness.
Bard 1, cleric/oracle 1, druid 1, witch 1.
1 person-day, this stretches a 40kg sack of rice to over a thousand person-days, and is ~always available.

Allfood.
Ranger 2.
5 person-days per caster level. Possibly also a way to remove non-magical but annoying materials, if you can eat them.

Harvest Season.
Druid 2, shaman 2.
1 person-day per caster level.

Alpha Instinct.
Bard 3, druid 2, medium 2, mesmerist 3, ranger 2, shaman 2.
1 + 1 person-day per level, assuming you’re in a forest it’s easy to forage in. Takes a while, can’t be done while moving (but can while resting.)

Create Food and Water.
Cleric/oracle 3, shaman 3.
3 person-days per caster level.

Bountiful Banquet.
Bard 4, cleric 4, druid 4, shaman 4.
2 person-days per caster level. But it mentions four courses, so potentially more than that in practice.

Fairy Ring Retreat.
Druid 7, shaman 7, witch 7.
4 person-days per caster level.

Mage’s Magnificent Mansion.
Sorcerer/wizard 7.
12 person-days per caster level, but each person-day is nine courses and that seems quite excessive. Assuming up to 108 person-days per caster level, but depends on interpretation.

Create Demiplane.
Cleric/oracle 8, sorcerer/wizard 8, summoner 6, witch 8.
10 person-days per level squared, but requires two casts to initiate food production. Can be made permanent with 20k gp, in which case it feeds 5 people per level every day.
The lesser variation is either seventh-level or fifth-level. If initiated with a normal casting, 3 person-days per level squared.
The greater variation can permanently double speed of production with a single casting and otherwise produces 20 person-days per level squared, and is always ninth-level.
Unlike the other options, requires a lot of extraction. Plane Shifts are expensive, and it’s uncertain how the food can be gathered. But it is a lot of food, and if an eight

Notes: a 5th-level cleric with 16 Wisdom can feed 37 people alive indefinitely. Any caster can, at a minimum, feed a number of people equal to the number of spell slots they have indefinitely. A demiplane-capable caster can easily feed ten thousand people if they’re capable of Plane Shifting and teleporting that much food, which they should be – there are teleportation spells which teleport any creature and all that they are carrying if it’s not more than a heavy load, so Ant Haul plus Strength bonuses should easily get you there. A 7th-level cleric at the same Wisdom can feed 93 to 128 people, depending on interpretation.
A 5th level druid feeds 45 to 50 people indefinitely, depending on conditions. A 7th level one feeds 88 to 130, depending on the interpretation. Overall, they’re surprisingly comparable – druid not having access to Create Food and Water is a significant downgrade for them, that spell is very efficient.

For gaining information.

Information is very valuable. These are some of the better ways to go about getting it.

Abadar’s Truthelling.
Cleric 1, inquisitor 1, oracle 1, paladin 1, warpriest 1 (Abadar).
Compel anyone to tell the truth. Except it has a visual effect, so you can see if someone made the save.

Heightened Awareness.
Alchemist 1, arcanist 1, bard 1, druid 1, hunter 1, inquisitor 1, investigator 1, medium 1, mesmerist 1, occultist 1, psychic 1, ranger 1, shaman 1, skald 1, sorcerer 1, wizard 1.
+2 competence on all trained Knowledges and Perception.

Know the Enemy.
Cleric 1, hunter 1, inquisitor 1, oracle 1, paladin 1, ranger 1, warpriest 1.
+10 insight to any knowledge check regarding a creature type you’ve encountered within the last 24 hours. Should almost always work with, at the very least, Knowledge (local).

Whispering Lore.
Cleric 2, druid 1, hunter 1, medium 1, oracle 2, psychic 1, ranger 1, warpriest 2, witch 1 (elf).
+4 insight to Knowledge checks depending on terrain.

Page-Bound Epiphany.
Arcanist 2, bard 2, cleric 2, magus 2, oracle 2, skald 2, sorcerer 2, warpriest 2, wizard 2.
+1 circumstance to a Knowledge check per caster level, up to 10.

Commune with Birds.
Arcanist 2, bard 2, druid 1, hunter 1, mesmerist 2, psychic 2, ranger 1, shaman 2, skald 2, sorcerer 2, witch 2, wizard 2 (tengu).
Learn anything that a bird within a mile of you would know. Very good if you need to know something and there’s birds in the area, which there usually are.

Ancestral Communion.
Bard 2, cleric 2, oracle 2, skald 2, warpriest 2.
+4 to +8 insight, depending on level, on up to 10 Knowledge checks per level.

Hunter’s Lore.
Hunter 2, inquisitor 2, ranger 2.
Take 20 on skill checks to identify a foe’s weaknesse.

Augury.
Cleric 2, medium 2, occultist 2, oracle 2, psychic 2, sahirafiyun 2, shaman 2, spiritualist 2, warpriest 2, witch 2.
Learn weal/woe/weal and woe/nothing about the next half-hour. Can’t be retried by the same caster. Very good on account of being very cheap.

Discovery Torch.
Bard 2, cleric 3, inquisitor 2, oracle 3, skald 2, warpriest 3.
+2 enhancement on Knowledge to identify monsters in the area, as well as Perception and Sense Motive.

Create Treasure Map.
Bard 2, druid 3, psychic 2, ranger 2, sorcerer/wizard 2. 100gp.
Requires a dead creature, but you learn one source of treasure per three caster levels the creature knew of while it was still alive, with no save and perfect accuracy.

Gentle Repose.
Cleric/oracle 2, shaman 2, sorcerer/wizard 3, witch 2.
For making stuff like Create Treasure Map more useful.

Blood Biography.
Arcanist 3, bard 2, bloodrager 3, cleric 3, inquisitor 3, medium 2, occultist 2, oracle 3, psychic 3, skald 2, sorcerer 3, warpriest 3, wizard 3.
Identify anyone from a drop of blood. They only get a save if they’re still alive or undead. Also tells you how the blood was shed.

Speak with Dead.
Cleric/oracle 3, inquisitor 3, medium 2, psychic 3, shaman 3, spiritualist 3, witch 3.
One question per two caster levels, guarantees truthfullness if they’re the exact same alignment you are. (So only prefer this over Blood Biography in cases where alignment is certain or you really need specific questions answered.)

Akashic Communion.
Arcanist 4, bard 3, medium 3, mesmerist 4, occultist 3, psychic 3, shaman 3, skald 3, sorcerer 4, spiritualist 4, witch 3, wizard 4. 100gp for non-divine casters.
+10 insight bonus to up to 1 Knowledge check per three caster levels, always treated as trained.

Commune.
Adept 5, cleric 5, inquisitor 5, occultist 5, oracle 5, shaman 5, warpriest 5. 500gp.
Ask yes/no questions of your deity. Extraordinarily powerful. See here for an in-universe explanation of how to best use it, though it’s not like you need to learn Huffman coding to do it.

Commune with Nature.
Druid 5, hunter 4, psychic 5, ranger 4, shaman 5.
Learn a lot of things in a very, very large area. Signs of the Land is a third-level variation that doesn’t give the most useful bits of information, but can still be very convenient.

Commune with Plane.
Cleric 5, hunter 4, oracle 5, psychic 5, ranger 4, summoner 5, summoner (unchained) 5, warpriest 5, witch 5.
Like Commune with Nature, but only useful outside the Material.

For boosting skill checks.

This doesn’t include Knowledge-only checks, those go in the information section.
If you need to give an important speech, doing it with a modifier of +20 rather than +5 can be extremely valuable. Sometimes, the person who played the vital role in a conflict just needs to do that, and you can’t outsource the task. And if you are the party face, +40 rather than +20 is also pretty high-value.
Extrapolate to skill checks other than Diplomacy.

Guidance.
Adept 0, cleric 0, druid 0, hunter 0, inquisitor 0, medium 0, occultist 0, oracle 0, shaman 0, spiritualist 0, summoner 0, summoner (unchained) 0, warpriest 0, witch 0.
Use it constantly. +1 competence to everything adds up over time.

Ceremony.
Cleric 1, oracle 1, warpriest 1. Lots of different variations, some are good. Just check if your domains allow for anything handy.

Innocence.
Bard 1, mesmerist 1, skald 1.
+10 competence to Bluff checks to convince people of your innocence.

Moment of Greatness.
Arcanist 1, bard 1, cleric 1, oracle 1, psychic 1, skald 1, sorcerer 1, warpriest 1, wizard 1.
Doubles the value of a morale bonus.

Fallback Strategy.
Cleric 1, inquisitor 1, magus 1, oracle 1, paladin 1, warpriest 1 (Torag).
Reroll a single skill check.

Crafter’s Fortune.
Alchemist 1, arcanist 1, investigator 1, occultist 1, red mantis assassin 1, sorcerer 1, wizard 1.
+5 luck on next Craft check.

Skim.
Alchemist 1, arcanist 1, bard 1, cleric 1, inquisitor 1, investigator 1, magus 1, occultist 1, oracle 1, psychic 1, shaman 1, skald 1, sorcerer 1, warpriest 1, witch 1, wizard 1.
Read four times faster.

Tap Inner Beauty.
Bard 1, cleric 1, oracle 1, skald 1, warpriest 1, witch 1 (Shelyn).
+2 insight to all Charisma-related checks.

True Skill.
Alchemist 1, arcanist 1, bard 1, investigator 1, magus 1, medium 1, psychic 1, skald 1, sorcerer 1, wizard 1.
Insight bonus equal to 1/2 caster level on a single skill check.

Adoration.
Arcanist 2, bard 1, mesmerist 1, psychic 2, skald 1, sorcerer 2, witch 2, wizard 2.
+2 morale on Diplomacy checks made to influence creatures.

Seducer’s Eyes.
Arcanist 2, bard 2, skald 2, sorcerer 2, witch 2, wizard 2 (Calistria).
+1 to +5 untyped on Charisma-based skill checks when interacting with those who might conceivably find you sexually attractive. This should at least half of the population as a baseline.

Fox’s Cunning.
Alchemist 2, arcanist 2, bard 2, investigator 2, medium 2, psychic 2, redmantisassassin 2, skald 2, sorcerer 2, summoner 2, summoner (unchained) 2, wizard 2.
+4 Intelligence for the duration. See also Eagle’s Splendor, Owl’s Wisdom, Bear’s Endurance, Bull’s Strength, and Cat’s Grace.

Bestow Insight.
Arcanist 2, bard 2, cleric 3, inquisitor 2, medium 2, mesmerist 2, oracle 3, psychic 2, skald 2, sorcerer 2, warpriest 3, witch 2, wizard 2 (human).
+2 to +6 insight, and you get to reroll and take the higher roll once. Hire someone else to use it on you if you want to be considered trained – some cleric will have ranks in whatever skil it is you need. Otherwise, you can just cast this on yourself.

Cleromancy.
Arcanist 3, cleric 2, druid 3, hunter 3, inquisitor 3, oracle 2, sorcerer 3, warpriest 2, wizard 3.
Up to +4 luck to any d20 roll. A bit clunky to use, the duration is quite low.

Heroism.
Alchemist 3, arcanist 3, bard 2, bloodrager 3, inquisitor 3, investigator 3, medium 2, psychic 3, skald 2, sorcerer 3, spiritualist 3, summoner 3, summoner (unchained) 3, witch 3, wizard 3.
+2 morale to everything.

Aura of Cannibalism.
Arcanist 3, cleric 3, oracle 3, sorcerer 3, warpriest 3, witch 3, wizard 3.
+2 competence to skill checks.

Deadly Juggernaut.
Antipaladin 3, cleric 3, inquisitor 3, oracle 3, paladin 3, warpriest 3.
+5 luck to Strength-based skill checks, but has some really inconvenient setup.

Planned Assault.
Cleric 3, hunter 4, oracle 3, paladin 3, ranger 4, warpriest 3 (Angradd).
+2 to +6 sacred/morale, the description is contradictory. Probably sacred, though.

Untold Wonder.
Arcanist 5, bard 4, cleric 5, mesmerist 4, oracle 5, paladin 3, psychic 5, skald 4, sorcerer 5, warpriest 5, wizard 5.
Penalties from emotion effects are instead treated as morale bonuses equal to half their normal strength.

Complete buffstacks.

Knowledge: At 1st level, +1 competence from Guidance, up to +10 insight for specific types with Know the Enemy or Whispering Lore, potentially a +2 competence from Heightened Awareness.
At 3rd level, Ancestral Communion for +4 insight, +3 circumstance from Page-Bound Epiphany, +4 luck from Cleromancy. +11 total.
At 5th level, Akashic Communion for +10 insight, +2 competence from Heightened Awareness, +5 circumstance from Page-Bound Epiphany, +2 sacred from Planned Assault, +4 luck from Cleromancy, +2 Intelligence from Fox’s Cunning, +23 total. Potentially Heroism and Moment of Greatness, for +27 total.
At 7th level, Akashic Communion for +10 insight, +2 competence from Heightened Awareness, +7 circumstance from Page-Bound Epiphany, +2 sacred from Planned Assault, +4 luck from Cleromancy, +2 Intelligence from Fox’s Cunning, +4 from Heroism and Moment of Greatness, for +29 total.
You can also add some rerolls, just in case. It should be rare that you use all of these at once, but it’s good to be aware of the fact that you have the option.

There’s also the bare, cleric-only variation.
At 1st, unchanged.
At 3rd, unchanged.
At 5th, +3 insight from Bestow Insight, +1 reroll from the same spell, +5 circumstance from Page-Bound Epiphany, +4 luck from Cleromancy, +2 sacred from Planned Assault, and +2 competence from Heightened Awareness, for +16 total. Considering any cleric who knows those spells exist can just spend a day doing this, it’s really quite impressive!

Diplomacy:
At 1st level, +1 competence from Guidance, +2 insight from Tap Inner Beauty, for +3 total. At 3rd, you get Cleromancy for +4 luck and Eagle’s Splendor for +2 Charisma, for +9 total.
At 5th, you can add Planned Assault for +2 sacred and +11 total.
That’s kinda it, but it still makes even completely inept clerics fairly good at Diplomancy in a pinch. And if they can get any real help, Heroism + Moment of Greatness is +4 more. Fallback Strategy is always available.