Kes seemed to have the most story potential, but the writers just couldn't figure out what to do with it.
Seven was a dynamic character that went through an interesting character arc. It would've been a lot easier to take her seriously if they didn't have her parading around in a outfit that set women's rights back 200 years.
I was disappointed to see Kes go, but for better or worse it's what happened. The Seven character did give the show a much needed reboot of sorts.