Xillia was my entry into the franchise, and there are so many fantastic entries! I haven't played all of them, but I've played most of them since the PS3-onwards.
I love pretty much all of the ones I've played, but I always go back to Zestiria & Berseria as my favourites. I think it's the way they complement eachother. That every time I play through one, it helps me appreciate the other even more.
Both are also prime examples to me of how excellently Tales handles its stories and character development, like few other RPGs manage to do to the same degree.
Berseria also has the character card minigame, and that is probably the easiest minigame for me to get completely lost in for hours.