I'm surprised to see a lack of topics for this game series. To be honest, I haven't played Tales of Symphonia (the game that seemed to get people into the series in the first place.), but I've played Tales of Graces (F) and I'm currently playing through Tales of Xillia.
For my first Tales Of game, Graces was one of the best Japanese role-playing games I've ever played. Sure the story is about friendship and stopping your possessed friend from destroying the world, but the gameplay was beautiful. My only nitpick is that some of the bosses were unbalanced (The final boss in the epilogue arc happens to be one of those). Otherwise, there was a large amount of stuff to do in the game (even with a linear overworld) and the combat was fun.
Xillia is shaping up to have a more interesting storyline, but it doesn't feel as easy to put together combos. But then again, Graces was like that for a while.