For me, Across the Nightingale Floor is the best of the Otori books, simply because that is where I fell in love with the incredible world Lian Hearn has created. Her characters are so realistic, genuinely flawed and honest. This series definitely falls into the category of "epic".
It's not a light read, the content can at times be quite graphic and emotionally wrenching, but it it all the same beautiful and complete. I've never read anything quite like it.