I voted for Seven.
Despite the initial reasons for casting Jeri Ryan, I felt that she did a superb job acting wise and she also had some strong material to work with. I did also like Kes, and Jennifer Lien did a good enough job. It would've been interesting to watch her powers develop and to see her grow into old age over the course of the series. She definately had more potential than a rookie Ops officer and a wannabe Ambassador and bad cook.