Basically, what title says. To be specific, there are some points to define what is considered "best" here:
2-3 spell level
- becomes available more or less early;
- daze effect still lasts more than 'till the beginning of your next turn', freeing some of your turns for things other then 'I cast Dazing X spell!';
- relatively high number of spell slots (if nothing else, you may always apply Heighten Spell, if you need a level to be higher for some reason);
- lesser metamagic rods are applicable here.
Allows a save, no attack rolls, no SR (reflex is the best, will or no save are a close second)
- in the end, your combination will allow a save in any case, so it is better, if initial spell allows a save, than if it allows spell resistance or if it requires an attack roll.
- reflex save is usually a lowest one, and fortitude is highest (enormously high or even akin to immunity sometimes).
Affects as many targets as possible
- area effect is probably better than multi-target spell, because of vision and duplicates and etc;
- if it is an area effect, instantaneous duration is probably the best (thanks to Selective Spell feat);
- ... or, you may consider something like Incendiary Cloud, which may affect additional targets over time, not sure if it is worth it.
Reliable damage type
- force and untyed are certainly best types;
- I'm not sure what is better between commonly resisted types, like physical and energy attacks (still, my guess, sonic is the best here).
Those are my thoughts. If you see, I'm wrong somewhere, you are encouraged to let me know it in your answer.
And, if you are interested, this is for my real game. For now, I play an Arcanist class and use Stone Call, but I think I will have hard time beating through damage reduction somewhere along next module. So I seek an alternative. My initial plan was to add Havoc of Society to my Stone Call, but now I see, I should read descriptions better. As I am an Arcanist, I am generally interested in spells, present on Sorcerer/Wizard spell list. But bonus points for pointing out proper spells for other classes, as I am interested in general solution as well.