I found Kes completely bland, though she was treated very badly with the Neelix Is Jealous scenes they had for a while.
Kes was starting to get a little interesting as she developed her powers, but at the same time, they were too strong to have the character realistically kept around.
Seven was a fantastic character, so well developed over the years (final romance aside) and brilliantly played. I'll never forget Jeri Ryan playing all those different characters, including a Ferengi. The only problems with Seven were the complete overload of her in season five, and the use of her as a deus ex machina to resolve so many episodes.
Kim: I'm detecting some weird technobabble.
Janeway: A possibly dangerous anomaly that we know nothing about?
Kim: Yeah. I suppose we should steer clear.
Janeway: Ha! Good one, Ensign. Take us in, Mr. Paris.