I think it might be a tie between Karma and Sona. Karma isn't very popular so I don't have a lot of experience to speak from, but her kit just feels a little underwhelming. It seems like it's trying to be the jack of all trades and therefore falls a little flat. On the other hand, I used to really like Sona, but now she suffers the same problem as Karma. Not to mention if the player isn't being smart, her fragile base stats do not help.