Kes was more interesting because she was more different from everyone else. At the beginning Chakotay's only role was to tell Maquis not to rebel, at the end his only role was to tell Janeway when she was going zany. His only development was stereotypical mysticism.
Kes started out as an innocent, but what if she struggled to maintain that innocence as she started to gain more and more telekinetic power? That's a missed opportunity there.
Story of Voyager character development, I guess. The only characters who really were used to their full potential were Janeway, Doctor and Seven.
And I suppose Neelix, since he had no potential.