I recommend the Heian era.
Because the basics of modern Japanese culture begin from this time.
Japan before this time made a model of Chinese Law Code system,
However, Japan improved (Ritsuryo) Law Code system in the Heian era and became the new politics system.
A samurai was born from a noble militarily in this time.
"The Tale of Genji" "Tosa diaries" are the bases of Otaku culture.