Grammar books say that 'quisque' takes 'very often' a plural verb.
And I saw two sentences which takes plural or singular verb.
Both are acceptable?
sibi quisque habeant quod suum est. (Plau. Curculio 1,3)
Ego, quid ad te tuorum quisque necessariorum scribat, nescio. (Ad Fam. 1,9,25)
Then, 'scribant' can also be used instead of 'scribat'?
And, the phrase 'optimus quisque' means 'the very ablest men (women,too? or should be optima?)'.
Then, how should I say for 'the very ablest man (woman) as a singular?
Actually the pronoun “quisque” takes very often a singular verb which agrees with this singular pronoun.
Anyway, “quisque” can also have the so-called “concordatio ad sensum”, i.e. a grammatical connection according to sense where agreement is determined by sense, and then the hortatory subjunctive “habeant” in “sibi quisque habeant quod suum est” (Plautus, Curculio, Act 1, scene 3, line 23) meaning “let each one of them keep what is his own “ is in the plural as it refers to “each one of THEM”.
Similarly in “Ego, quid ad te tuorum quisque necessariorum scribat, nescio” (Cicero, Ad Familiares, 1,9,25) the plural “scribant” could also be used instead of “scribat” as it could refer to the plural "tuorum...necessariorum".
In short, both plural and singular verb can be acceptable with “quisque”.
As for the idiomatic expression “optimus quisque“, literally meaning “every best man”, it is used to express universality and then means “the best men”/ ”all the best men”, while “optima quaeque” would mean “every best woman” and then “all the best women”.
Finally, for "the best man/woman)” as a singular you should say “Vir optimus” for a man and “Mulier optima” for a woman.
In this case, in fact, you cannot use “quisque”.