It matters not the class.
It matters not the class.
Something that is generally useful and doesn't compete for standard actions. The Restoration line is a good candidate.
Any spell that makes a permanent something from nothing would change society. (wall of stone or iron, create food and water)
Any spell that makes a useful permanent change or easy healing will do the same. (Shape stone, remove disease, raise dead)
Movement spells change things as well (levitate makes cranes, at will fly makes and air force, at will teleportation effects trade)
I think evocation is the least likely thing change the world but from and adventure stand point an at will blast is nice to have.
At will short term buffs are very powerful because they can be always up.
If i could pick one spell to have as caster at will based on fun I would go with some thing like scorching ray or magic missile.
I don't think the ability means what you think it means.
Abundent Casting (Ex) -Whenever you cast a spell that specifically targets a limited number of targets (such as “one creature/level” or “one or more creatures”), add your tier to the number of targets the spell can affect. For example, a 5th-level wizard/1st-tier archmage can target up to six creatures when casting haste (five fromcaster level 5th and one from 1st tier). Alternatively, you can expend one use of mythic power when casting a spell that affects only one target (such as “one creature”) to have the spell affect an additional target. If the spell requires you to succeed at a melee touch attack, you must hold the charge in order to make a second touch attack.
I did not realize there were multiple abilities of the same name. I am referring to the mythic archmage ability.
I'm assuming wish.
Probably strong, single-target spells that are normally balanced around affecting only one enemy. Depends on your level which is best, then.