I'll put in a vote for the Panini Sutra (Aṣṭādhyāyī) [1], which is a sort of "Backus–Naur form" for classical Sanskrit grammar. I don't understand Sandskrit, but I've heard from people that I respect that the Panini Sutra should be considered as one of humanities great intellectual achievements.

[1] https://en.wikipedia.org/wiki/A%E1%B9%A3%E1%B9%AD%C4%81dhy%C...