Every genre has its Citizen Kane, whether in anime or hentai. Most would point to the Loli genre with the 1984 six-part OVA series Lolita Anime as its Citizen Kane. While it holds significance with its place in history, it has not aged well with the modern era other than being a time capsule. If there is one OVA that I think truly deserves to be crowned that accolade, Shoujo Ramune would be the one to fulfill that position, no questions asked.