Boy, that's a tough one! I'd certainly have to consider India, with the Mahabharta, the Bhagavad Gita and such being 5,000 years old. And China has had phenomenal literature since ancient times, too.
The aboriginal tribes of Australia have living paintings, still added to each year, that tell the story of their tribe for 20,000 years. That's certainly rich.
Of course, if you limit things to a couple of centuries, then many other nations come into play.
I don't think that there is any way to compare richness across cultures!