I'm tied between Ventus and Aqua. I've faced and defeated Vanitas's Lingering Spirit and Mysterious Figure with Aqua so far, but with Ven, I can end most encounters quickly. His Faith move is very useful on normal enemies. He also gets a lot of useful Shotlocks early in his game.
Aqua is also good because she has a wide selection of magic, and a lot of it is useful. Not to mention her block surrounds her from all sides. Like Ven, her dodge move is short-range, allowing control where you can go, unlike Terra.
That's my nitpick with him, BTW. His Slide move is nice from one area to the next, but its actually implementation as a dodging move is questionable. Even maxed out, I find myself getting hit by mashing the button without Sonic Impact installed. Also, he's pretty slow and has a bad recovery time. On the upside, his personal commands are extremely powerful with pretty good Command Styles.