Abyss > Berseria > Xillia 2 > Graces > Vesperia > Xillia > Hearts > Symphonia > Legendia > Symphonia 2 > Zestiria
Still need to go through the pre-Symphonia stuff at some point.
I consider Symphonia to be the worst of the good Tales games. I look on it fondly because it was my introduction to the series, which has become my absolute favorite game series, but everything about it is pretty middling. Every game above it on that ranking does at least one thing significantly better. Its character cast is average, the story is pretty generic, the music is forgettable, and the battle gameplay is pretty simplistic especially compared to Graces or Hearts.
It's still a 7.5/10ish game, but that puts it solidly below all the 8s and 9s that Tales typically puts out.
And re:Zestiria, that game is just bad all the way through. Not "bad for Tales standards" or "good to a certain point when it all falls apart" or "bad but with some good ideas." Bad all the way around and it will forever be a black mark on the series. And since it's so connected to Berseria, they can't even retroactively call it a spinoff like they did to try and erase Tempest from people's memories.