Symphonia was my first Tales game and I really enjoyed it, then I played most of the rest (not the DS games or PSP ones) and realized that Symphonia was the only one I actually liked, or really I just got bored after playing middling game after game. I was hesitant on getting Vesperia, but after IU let me down I picked it up and now I consider that the best Tales game.
The Tales series is really weird for me. I think if you play the best, you're better off not touching the rest, and Vesperia is the current best (UNTIL TALES OF TEN FOR WII!!! LOL). Vesperia has the best combat, dialogue, and characters.