Someone finally put up an FAQ of Ys on gamefaqs, so I better get started. I like to play with FAQs, as I hate missing anything, and I don't tend to replay my JRPGs.
As for Xillia being the best Tales, not even close...
For me, the best one story-wise is Vesperia (Yuri Lowell is quite possibly the best main protagonist in any JRPG ever), and gameplay-wise, Graces F. I loved Phantasia, Destiny, Eternia, and Abyss. I didn't give Symphonia a real chance back in the day, so I'll be getting the HD port when it comes out soon.
I loved Xillia, but man, what a horrible level design. The environments were sooooo bland, samey, and super linear. Only a few places were nice to look at, like the town Jude and Milla met. Pretty much every field map was the same. All the seahavens ARE the same.
If there is one thing I love anbout JRPGs, it's the world they take place in. The vistas, the locales, etc. Xillia felt like I was in the same places throughout the whole game, only in different seasons. No real variation at all.