Seven is my favorite character ever but I don't know if I'd say it was she who made Voyager better. Voyager had started to get better at the end of season 3 when the writing suddenly improved drastically after the disaster that was "Favorite Son". The writers were pulling their sh*t together before they the Kes-Seven switch, Seven's coming on just happened to coincide with it. Nonetheless, I don't know if I'd have enjoyed Voyager as much without Seven. But I know for sure that I would have enjoyed Voyager even more if Kes had been kept on in addition to Seven joining. Voyager had a chance to really do something a bit new if they jettisoned a male cast member and kept Kes - have a much more female-centric show than the others for reasons beyond simply the captain being female. Voyager had chances to do a lot of things which it didn't unfortunately.