It ties with Symphonia. Symphonia has a special place in my heart, although I am aware of the terrible character flaws with the main protagonist now. Thankfully, I realized what a hypocrite he is. Abyss, on the other hand, shines out to me because the character development is so large - particularly for the main protagonist.
I have yet to find a Tales game that I'd enjoy playing as much as these two that also that has much character development in it that I care about the characters.
Cloud and Sephiroth - battling each other since '97 for your amusement Give it a rest already!
Vesperia introduced me to the series, and I can honestly say, I like everyone in the party. I can't think of any other games I've played where I don't dislike/hate anyone, aside from Vesperia's. This game and Abyss are the only Tales I've logged at least 300 hours on, and I've yet to burn myself out on them. Only game in the list I haven't replayed is Innocence.
Can't even get myself to add DOTNW on the list; 13 hours of my life I'm never getting back.