One thing that I think that we should consider when we are comparing the characters and the actresses are the roles
the characters were supposed to have in the show.
Kes was supposed to be more of a background main character. If we look at the first episodes, the foreground main character was Janeway with Chakotay as no. 2 while Paris, Tuvok and Torres came straight after. The Doctor, Kes, Neelix and Kim were more like background main characters.
That changed later on when The Doctor got a more prominent role
However, that meant that Kes was kept in the background in many episodes but when she was given a place under the spotlight, we could actually see that the character had a lot of quality. The same for Jennifer Lien's acting. When she was given the chance to shine, she took it.
When Seven was brought in, her role was immediately one of the foreground main characters (if not the
main character). The show became focused on three characters: Seven, The Doctor and Janeway while the others were in a way shoved in the background. That gave Seven a lot of screen time and it also gave Ryan the opportunity to show her skills more than the amount of screen time that was given to Jennifer Lien.
Therefore it's hard to compare the characters and the actresses. They were very different and had such different roles.