Aqua

His initial plan had been Terra. A young man who was healthy and strong like the earth he was named after. Of course when he noticed young Master Aqua's performance, he reconsidered.

For one strength diminished with time but magical prowess only increased. If he took Terra's body then eventually he would get old and weaker once more. But if he took a powerful magic user's body then that wouldn't happen as quickly.

So it was decided that he'd take Aqua's body. She was a little harder to manipulste than Terra but not all too impossible. She had a little more ego to work with and of course she was still affectionate to Ven and Terra, a fact that was all too exploitable to Xehanort.

And in the end....he had done it.

Xehanort looked at his or rather her new form. It was Aqua's body but several of their features had overwritten hers. Now her skin wasn't a pale white but rather a chocolste brown. Her aqua blue hair was now snow white and her once deep sapphire eyes were a burning amber.

All in all she was impressed by how easy it had been. As Aquanort stretched her stolen limbs she could feel the youth and vigor in her body.

Now it was only a matter of figuring out what to do next.