Tales of the Abyss.
I adore the characters, the scenario, the battle system, the controls... just everything. I've poured well over 300 hours into it, perfecting my file, doing every sidequest, etc. It's fun on your own and even more fun to play through with a friend which can change everything up. Sure, after that much time spent on it, I've gotten a little sick of it, but even my other top favourites just... they aren't the same. So ToA gets top pick.
Honourable mentions to Pokémon, Kingdom Hearts (COM specifically), and The World Ends With You.