Tales and Skies, by far, are the best on the Cube. In fact, those two and Golden Sun are my favorite RPGs of all time.
The battle system in Tales makes the purchase immediatley worth it. It's not turn-based at all- you control one character, and you target one enemy and attack on a 2D plane. Different button combinations perform different moves. Everything is pretty much customizable, including the different strategies your team mates use (for instance, you can set one person to attack from a distance, to attack the enemy attacking your weakest team mate, and to attack only usuing magic until his/her mp is down to 1/4). You can also tell your team mates to do a certain move at any time, so you still have control over them. It's awesome.
Skies has a more traditional battle system, but it still has a few unique inclusions such as spirit points. I don't want to go into a ton of detail, but there are a lot of different things to try out while fighting. I'm not the biggest fan of turn-based battling, but something about Skies makes it exciting and actually FUN. Plus, it's pirate themed, so it's automaticaly cool. There are quite a number of things to do outside of the story, such as finding discoveries and defeating other famous air pirates (for a reward, of course).
Both adventures are pretty lengthy, and you'll probably get over 100 hours out of those two combined.