I'm curious about the utility of counterspelling with low level spells, if one chooses to feat for it. Particularly the feasibility of keeping low-level spells prepared specifically for use as counters. The rule for counterspell in Pathfinder 2E says:
[Witch, Wizard] You expend a prepared spell to counter the triggering creature’s casting of that same spell. You lose your spell slot as if you had cast the triggering spell. You then attempt to counteract the triggering spell.
Note that the countering spell could theoretically be a lower-level version of the triggering spell.
If a Wizard further takes the Clever Counterspell feat, they can use a different spell (again not necessarily of the same level):
At the GM’s discretion, you can instead use a spell that has an opposing trait or that otherwise logically would counter the triggering spell (such as using a cold or water spell to counter fireball or using remove fear to counter a fear spell). Regardless of what spell you expend, you take a –2 penalty to your counteract check, though the GM can waive this penalty if the expended spell is especially appropriate.
The rule for counteract checks appears to be the following (non-spell portions edited out)
When attempting a counteract check, add the relevant skill modifier or other appropriate modifier to your check against the target’s DC. ... use the caster’s DC. ... the counteract check modifier is your spellcasting ability modifier plus your spellcasting proficiency bonus, plus any bonuses and penalties that specifically apply to counteract checks. If an effect is a spell, its level is the counteract level.
Critical Success - Counteract the target if its counteract level is no more than 3 levels higher than your effect’s counteract level.
Say we have a Wizard who of course started his career at level 1 with 18 Int, and raises it at every attribute raising level. Now say both the Wizard and a baddie meet up at level 5. Baddie tries to cast fireball. The GM rules (as per above) that an ice spell can be used to counter it. The Wizard happens to have the level 1 spell Snowball prepared. What are the Wizard's odds of getting that Critical Success he needs?
Now say they meet again at level 20. Baddie heightens his fireball to a 10th level spell. Our Wizard, knowing he could face 10th level spells, has Snowball prepared into a 7th level spot. Now what are the Wizard's odds of counteracting that fireball spell cast?