The find familiar text states:
Finally, when you cast a spell with a range of touch, your familiar can deliver the spell as if it had cast the spell. Your familiar must be within 100 feet of you, and it must use its reaction to deliver the spell when you cast it. If the spell requires an attack roll, you use your attack modifier for the roll.
The first sentence of counterspell states:
You attempt to interrupt a creature in the process of casting a spell.
Of course, the familiar is not "casting the spell," because familiars do not themselves have spell slots, but the find familiar text says that it delivers the spell "as if it had cast the spell."
Does this mean that casting a spell through a familiar this way means the familiar can be targeted to counterspell the spell, even though the familiar itself is not using the Cast a Spell action, because it's treated as if it was casting the spell?
Or would counterspell only be able to be cast on the person actually expending a spell slot to use the Cast a Spell action, i.e. the wizard itself, not the familiar?
Best Answer
No, counterspell must target the caster casting the spell to be countered, not the familiar through which the spell would be delivered.
The description of find familiar says "your familiar can deliver the spell as if it had cast the spell." The key words here are "as if." The familiar is delivering the spell "as if" it had cast the spell, but that does not mean it actually cast the spell.
Examining the core rulebooks reveals that the designers consistently use "as if" to signal rules of like treatment, not identicalness. For example:
By the same logic, for purposes of delivering the spell your familiar receives the treatment it would receive if it had cast the spell, but it has not actually cast the spell. Counterspell therefore would not be effective if it targeted the familiar rather than you.