Attic, the highest prestige dialect, hands down. For centuries, this has been the mainstay of Greek language education in Europe.
There is no better reminder of this than Newton's Trinity College Notebooks taken down in serviceable, not perfect, classic Attic. (Note he uses Attic syntax, verbs in 3rd person singular, matching a plural subject.)
Of course, works of Italiot authors, like Archimedes, were written in Doric, but have not survived, and all extant excerpts in our possession were "translated" to Attic in antiquity. Aristotle's books and Euclid's elements are basically in Attic, virtually fully overlapping with Koine, and serving as its cynosure; and fully comprehensible to educated Koine readers, then and now. Even Diophantus, 3rd century AD, writes in Classical language, the language of all mathematical texts he'd read, and not in Koine, the language he probably spoke at home.
The Ancient Greek taught in Greece today is Classical (Attic), with some Homeric. Koine need not be taught as "Ancient Greek" there, as Testament (funny Koine) Greek is used in churches every day, and underlies the reconstructed formal language, Καθαρεύουσα, used for higher education and state functions for close to two hundred years; much of this language was taught as "modern" Greek. Panegyric school speeches were given in it, arguably comprehensible to students. It was abandoned only recently in the mainstream, but still lingers in the culture; as English incorporates Shakespeare maxims, so does colloquial Greek run on Testament maxims. Wonderful 150 year old works of literature in Koine/Καθαρεύουσα have started being translated to colloquial Greek only recently.
In Western Europe, and the US, "Ancient Greek" textbooks have, almost exclusively, been covering Classical (Attic) Greek, given the bulk of the corpus of Greek authors; with a smattering of Homeric. I think Biblical scholars in the last century have splintered off and are hyper focussing on Koine, but this is a side phenomenon.
(I know I might irritate people to scream with this, but you might think of Homeric as Robert of Gloucester, Classical as Chaucer, and Koine as Shakespeare.)