Lust Epidemic, Treasure of Nadia, The Genesis Order and now Symphony of the Serpent all follow the same story arc, or are rather set in the same world and the story has been progressing about the background story and "lore" through each game.
Treasure of Nadia is their best game and easily top 5 adult games ever made, imo easily number 1.
Lust Epidemic is also very good however the animations aren't quite as perfected yet still very good but you see a clear improvement going into ToN which not only has the best story imo but also animations.
The Genesis Order is also very good but still doesn't quite get better then Treasure of Nadia, potentially makes the top 5 list also though.