I've always thought that Kes is a bit of an underrated character. She had more potential than I think the writers recognised, as they never seemed to know quite what to do with her.
For that reason, Seven was a stronger character in that her arc was far more clearly defined, and reasonably interesting. Unfortunately, her progression to becoming more human is not unlike Data's, which to me always brings it back a bit. It also didn't help that so much of S4, and arguably the subsequent seasons, was devoted to her to the detriment of certain other characters who fell by the wayside.
For these reasons, although I think that Seven probably is a better character, I would vote for Kes.