Youjuu Senki -A.D. 2048- is not really a masterpiece at all, but without strong competitors I find it entertaining enough to be called VN of the September 1993.
A very weak month. But if choosing among ero-centered games, a mahjong game and an animated little masterpiece, I'd choose the last one. Houma Hunter Lime Dai 03 Wa is VN of August 1993.
Cosmology of Kyoto publication month is unknown, so I just put it to February that had relatively few titles. Cosmology of Kyoto is the only masterpiece of February 1993 and thus VN of the month.