What's the best offensive spell at each level for a Arcanist//Full Sneak-attacker at each spell level?

Question

Inspired by Effective sneak attacks with spells.

Sneak attacks with spells are pretty unusual, changing the expected damage of many spells, and thus their expected value, drastically. An sneak-attacking spellcaster, then, is going to want a very different set of 'blasting' spells than your standard spellcaster, which is the heart of this optimization exercise.

The character in question will be using the following gestalt build:

Arcanist 1//Ninja 1
Arcanist 2//Ninja 1/Snakebite Striker 1
Arcanist 3//Ninja 1/Snakebite Striker 1/Greensting Slayer 1
Arcanist 4//Ninja 1/Snakebite Striker 1/Greensting Slayer 1/Eldritch Poisoner 1
Arcanist 5//Ninja 1/Snakebite Striker 1/Greensting Slayer 1/Eldritch Poisoner 1/Field Agent 1
Arcanist 6//Ninja 1/.../Gray Warden 1
Arcanist 7//Ninja 1/.../Assassin 1
Arcanist 8//Ninja 1/.../Red Mantis Assasin 1
Arcanist 9//Ninja 1/.../Master Spy 1
Arcanist 10//Ninja 1/.../Deep Sea Pirate 1
Arcanist 11//Ninja 1/.../Sleepless Detective 1
Arcanist 12//Ninja 1/.../Twilight Talon 1
Arcanist 13//Ninja 1/.../Crimson Templar 1
Arcanist X//Ninja 1/.../Crimson Templar X-12

Most relevant is that from levels 1-12 the character is gaining 1d6 sneak attack dice per level, and then gains sneak attack dice at 14th, 17th, and 20th level, along with extra bonus sneak attack dice that only apply to evil outsiders who fail a save which is 16d6 at 16th level and goes up by 1d6 per level afterwards.

Given the character is attempting to maximize the damage dealt on average by their spells, which arcanist spell should be selected at each level if the Arcanist had not yet chosen a blasting spell and the spell so chosen were to be the Arcanist's damage workhorse?

Answer 1

Going off of your character build, we are eschewing Arcane Trickster (phew, easier maffs). The main assumptions I’m going to be making in choosing my spells here are:

A: We will always go first in combat, before allies and enemies.

B: We will always be within 30ft of the target.

C: The target is not immune to sneak attacks.

D: Multiple rays from a single spell all apply sneak attack damage separately.

E: We only have one target at any given time.

F: We hit all attacks.

G: An attack requires an attack roll to apply Sneak Attack damage (rules out Magic Missile)

H: Melee and Ranged are both okay, with favor being given to ranged spells if damage is equal.

I: We can only attack one enemy with a melee spell, we can attack as many enemies as we want with ranged spells

J: I will be using Intensify Spell + Potent Magic for all spells that do damage/level for the levels that it is relevant.

K: Maximize Spell affects only spell damage, Sneak Attack is still average

L: I am skipping over the lvl 15 feat that lets you use a free metamagic. It’d just let you cast double Maximized+Empowered blast spell.

M: Potent Magic CL boost does not apply to extra rays from spells.

N: I'm going to ignore your class choices and just go off of the "+1 sneak attack until level 12, then +1 sneak attack at 14, 17, and 20". I'll go into more detail after the breakdown.

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Character Level - Spell Name [Average Damage, floor, ceiling] (Runner-up [Average Damage, floor, ceiling])

1. Potent Magic Snowball [14,4,24] (Potent Magic Shocking Grasp [14,4,24])
2. Potent Magic Snowball [21,6,36] (Potent Magic Shocking Grasp [21,6,36])
3. Potent Magic Snowball [28,8,48] (Potent Magic Shocking Grasp [28,8,48])
4. Intensified Potent Magic Snowball [35,10,60] (Scorching Ray [28,8,48])
5. Intensified Potent Magic Snowball [42,12,72] (Scorching Ray [31.5,9,54])
6. Intensified Potent Magic Snowball [49,14,84] (Scorching Ray [35,10,60])
7. Scorching Ray [77,22,132] (Intensified Potent Magic Snowball [56,16,96])
8. Empowered Scorching Ray [98,28,168] + Quickened Cantrip [25,10,50] (Intensified Potent Magic Snowball [63,18,108])
9. Empowered Scorching Ray [105,30,180] + Quickened Cantrip [28,11,56] (Intensified Potent Magic Snowball [66.5,19,114])
10. Maximized Scorching Ray [118,68,168] + Quickened Snowball [52.5,15,90] (Intensified Maximized Snowball [95,70,120])
11. Maximized Scorching Ray [187.5,105,270] + Quickened Snowball [56,16,96] (Intensified Maximized Snowball [98.5,71,126])
12. Maximized Scorching Ray [198,108,288] + Quickened Scorching Ray [168,48,288] (Intensified Maximized Empowered Snowball [132,102,162])
13. Maximized Scorching Ray [198,108,288] + Quickened Scorching Ray [168,48,288] (Intensified Maximized Empowered Snowball [132,102,162])
14. Maximized Empowered Scorching Ray [244.5,108,342] + Quickened Scorching Ray [178.5,51,306]
15. Maximized Empowered Scorching Ray [244.5,108,342]
16. Maximized Empowered Scorching Ray [244.5,108,342]
17. Maximized Empowered Scorching Ray [248,111,360]
18. Maximized Empowered Scorching Ray [248,111,360]
19. Maximized Empowered Scorching Ray [248,111,360]
20. Maximized Empowered Scorching Ray [258.5,114,378]

There are a few problems with your classes. Greensting Striker uses swift action to gain sneak attack, which costs us Quickened spells. Field Agent can only get Sneak Attack at Field Agent 3. Gray Warden requires 2nd level divine spells. Red Mantis Assassin is super feat intensive. Blah blah blah.

Long story short, you're gonna need to do research and find classes that work properly with this build. If your GM is like me, he would rule that your classes all stack as adding their combined levels to determine your Sneak Attack dice, making it outright better to stick to one class than to split up between a lot (9 levels across 9 sneak attack classes would be functionally the same as 9 levels of Rogue for determining Sneak Attack dice). It'd be better to give up sneak attack dice for higher level class features (perhaps Vivisectionist Alchemist the full way, Mutagen to boost to-hit and AC is pretty great, along with the general utility of having alchemy).

I hope this answered the abstract portion of your question.