I came up with this because I learned from Tumblr that there’s apparently more than a hundred fanfics for The Epic of Gilgamesh on Ao3, and I think it’s poetic in a way how we’ve still never really gotten over the first story ever.
My answer would be Le Roman de Silence, from the early 1200s, which is about the adventures Silence, a boy who was born as a girl (that’s the author’s way of describing him)and who eventually becomes a knight. I just stumbled across it randomly but I fell in love with it after reading it the first time, and I think it’s beautiful how something 1000 years old can still be around and not have lost any of it’s capability for arousing our emotions.
I mean there’s a lot of old literature I appreciate. In that I admire the work, and I believe the author was a genius in their time, and I’m fascinated by the historical and artistic importance of the work. But if we’re talking literature that I unambiguously enjoy reading, not because I’m learning something or appreciating something but because the artistry just gets me and I enjoy myself, I would say the oldest is The Robbers by Friedrich Schiller, published 1781.