If you had asked me after Week 2, I would have definitely said Jamelia. I was expecting her to be good at Ballroom and brilliant at Latin, but her Waltz was just decent and her Cha Cha was technically poor. However, ever since then, she's improved far more than I was expecting her to, and I'm really enjoying her dancing now.
If you asked me now... I guess Ainsley, even though he's already out, because I found him quite underwhelming on the entertainment front, which was where I thought he'd thrive, and his technique really wasn't good in most of his dances. His Salsa is the exception to both of those however, and is still one of my favourite dances this series.
As for the others, I was expecting Georgia to be better at Ballroom than she is, Katie to be better at Latin than she is (not much better, but still), and both Kirsty and Peter to be better dancers overall than they are.