I haven't played XC2 but my faves in order are:

Xenosaga 3
Xenosaga 1
Xenoblade
Xenogears
Xenosaga 2

They're all good games, but Xenosaga had a large impact on me. It really got me in to psychology and philosophy which has undoubtedly shaped my world view in some ways.

Really these games have epic stories, which I think is their greatest strength.