What are the best spells for a multiclass spellcaster?

In your opinion, what do you think are some of the more worthwhile spell choices for a character who multiclasses into a spellcasting role? With limited expertise, and even fewer spell slots, picking the right spell is more important than ever. Which ones might you pick up and why?

Which class did they start in, and which did they multiclass into? A Fighter/Wizard would be a fair bit different than a Champion/Cleric, or a Druid/Primal Sorcerer.

Enlarge is a useful spell for some melee types. Heroism is another useful buff for anyone.

In general you're unlikely to want blast spells or other instantaneous effects.

My player retrained from warpriest to fighter/cleric (yeah, we stretched the retraining rules), so I know what he liked. The no-brainer was Shield (to go with his maul). For fighters, the question always seems to be what to do with that third action, so if you're going to use a cantrip in combat, it needs to take one action. Anything that takes two actions is a preparation or out-of-combat option.

The second choice after Shield was between: Detect Magic (but the druid could already do that). Forbidding Ward (can work if you get the drop on an enemy and cast just prior to entering combat). Guidance (you become immune for a little while after you use it, so you can't do it all the time). Light (again, druid). Message, and Stabilize. He ended up going with Guidance.

He mostly used shield when he didn't have to move to attack and attacked twice if he had to move.

I've been looking into using the Primal list (Druid MC or Sorcerer MC) to make a secondary healer. The following are the spells that seem applicable.

Cantrips
Guidance - Not directly related, but a nice boost to Medicine Skill rolls.
Stabilize - An obvious choice.

1st
Detect Poison - Probably not that useful for a secondary healer.
Heal - This is the big one.

2nd
Dispel Magic - Good for removing those pesky spells.
Gentle Repose - Hopefully you never need this one.
Remove Fear - Nice, but pretty specific.
Remove Paralysis - Nice, but pretty specific.
Restoration - Nicely versatile.
Restore Senses - Nice, but pretty specific.

3rd
Neutralize Poison - A solid choice.
Remove Disease - A solid choice.

4th
Freedom of Movement - Remove movement restrictions.
Vital Beacon - A nice choice for healing multiple wounded.

5th

6th
Field of Life - Decent mass healing. Damages Undead as well.
Stone to Flesh - Nice, but pretty specific.

7th
Regenerate - Temporary Deadpool powers. Solid spell.

8th
Moment of Renewal - Magical caffeine for the entire party.

Here is a similar breakdown for the Divine spell list. (Cleric MC or Sorcerer MC.)

Cantrips
Guidance - Not directly related, but a nice boost to Medicine Skill rolls.
Stabilize - An obvious choice.

1st
Detect Poison - Probably not that useful for a secondary healer.
Heal - This is the big one.
Spirit Link - For specialized cases only. Usually Heal is better.

2nd
Dispel Magic - Good for removing those pesky spells.
Gentle Repose - Hopefully you never need this one.
Remove Fear - Nice, but pretty specific.
Remove Paralysis - Nice, but pretty specific.
Restoration - Nicely versatile.
Restore Senses - Nice, but pretty specific.

3rd
Neutralize Poison - A solid choice.
Remove Disease - A solid choice.

4th
Freedom of Movement - Remove movement restrictions.
Remove Curse - Nice, but pretty specific.
Vital Beacon - A nice choice for healing multiple wounded.

5th
Breath of Life - A great spell to avoid the cost of Raise Dead.

6th
Field of Life - Decent mass healing. Damages Undead as well.
Raise Dead - Expensive, but this is a 'must have.'
Stone to Flesh - Nice, but pretty specific.

7th
Regenerate - Temporary Deadpool powers. Solid spell.

8th
Moment of Renewal - Magical caffeine for the entire party.

Spells that scale with the player's imagination are pretty good. That's mostly illusion spells, of course. Illusory Object and Illusory Creature can be used for everythin from concealment to distraction.

I like shield and Guidance.
If Bard then inspire competence is nice.

I'm surprised no one has mentioned it, but Haste is a verily solid choice for most builds. {Who says no to an additional Strike or Stride action.)

Comprehend Languages can be decently good if you are the face of the party, of course dependant on how many social situations your groups runs into, and how many language barriers they face {Language Barriers can shut down the face of the group in there tracks.)

At higher levels, Prying Eye may not be a bad idea, especially if the group is lacking a scout of some sort. {Minute casting time is not the worst thing in the world, especially when you can scout out +3,000 feet from relative safety. Knowledge of what is ahead of you, and what you may face can be life saving.)

Oh, and also Augury, because its nice to know how likely a plan is going to work, and make changes if need. Example=

<Standing outside Ancient Red Dragon's lair>
"Ok, so were going to attack the Red Dragon in his lair?"

"Yes"

"And we have only been adventuring for 3 months. And got out butts kicked last week by what we though was a troll, but turned out to be just a really ugly old dwarf, whom lost his teeth and couldn't speak clearly?"

"It's...oddly specific you would go into detail of a story I was there for, but yes."

"And our plan is to charge in there, yelling at the top of our lungs, and then, when we reach said dragon, the four of us are going to dog pile on it, in the hopes we overwhelm it with our sheer mass?"

"Yes, the Troll incident allowed us to work out the kinks for the dog pile plan."

"Okay, give me a couple of minutes." <Casts Augury> "Yeah just checked, this plan is mostly likely not going to work. No it didn't give a reason why. In fact, instead of the Woe result, the spell told me, and I quote 'No. Hell no. Are you trying to Punk me? Just....No. Actually, I change my mind. Yes, good results will happen. Obviously if this is the plan you four have come up with, you guys getting eaten will be a good result for everyone involved. This plan gets a Weal result with a capital W, now go with my blessing.’”

Someone point we out using true strike on a staff for Spellcasters which is clever. That could work well if you have one big 2 action point attack.

A reroll is worth roughly +5 to hit.

No, a reroll is about +3 or so. Give or take a fraction. I've done the math a few times, but don't have it in front of me, though.

James is right if you are considering only success/failure, but the exact crit success/crit fail mechanism used in PF2 complicates matters wildly. A further complication is that not all rolls even have crit fail effects...

The Wall Spells of lvl 4 and forward are pretty solid, they don't allow enemies saving throws against their effects.

Resilient Sphere is not only versatile but when used against a hostile creature even if it manage the saving throw they will need to spend an action to attack it, making them take MAP.

Synesthesia is another great spell even if the foe manages the save, Dazzled, chance of failing concentrate actions and clumsy 3 are nasty.

Heroism, such great buff.

Hyper Cognition, 6 recall knowledge in a row, tell me everything.

No, a reroll is about +3 or so. Give or take a fraction. I've done the math a few times, but don't have it in front of me, though.

It depends. On a count flip (11+ to succeed) it is indeed a +5, converting a 50% chance of success to 75%. Its less valuable the less you need it.

No, a reroll is about +3 or so. Give or take a fraction. I've done the math a few times, but don't have it in front of me, though.

Technically a reroll is worth more/less depending on the number on the dice you need to roll.

IIRC, if you need a 10, it's less than a +4, but rounds up to +4.