There are a lot of Atelier games out there, and Gust keeps adding to the lineup with new installments in the main series and spin-offs to keep track of. It is a lot to deal with. Also, the major installments tend to come out in trilogies, which share storylines and sometimes gameplay mechanics. One line might not be as interesting for specific players, but another could be a perfect fit.
Source: N4G PC Which Atelier trilogy should you play