1. Seven of Nine
I hope the irony of Star Trek’s worst-ever character being replaced by Star Trek’s best character is not lost on anyone. Because Seven wasn’t just the best Voyager had to offer, she was the best the franchise had to offer.
Non-human Star Trek characters are defined by their relationship to humanity. Do they want to be more human, like Data? Do they struggle with a dual existence, like Spock or Worf? Or do they not care for it, like Quark? Seven began as the least human character in the main cast, but as a human by birth, she was destined to become more human, as her individuality reasserted itself, whether she wanted to or not. This was truly original. The problem with Voyager was the lack of character development, but Seven had character development built in, and she did actually develop over the course of the show, which was very rewarding to watch.
The show markedly improved when Seven was introduced. Not all of that was Seven’s doing, but it wasn’t unrelated, her introduction was part of a new, more coherent identity for the show. That identity harkened back to what the show was meant to be; the mixed Maquis/Starfleet crew was meant to create conflict, but that premise was forgotten almost immediately, and we forgot who was Starfleet and who was Maquis. But Seven was truly other, truly different, and her presence created the conflict the show was always meant to have.
Not to mention her inner conflict. She was just a great character.
There was something more than a little problematic about how she was introduced for the sex appeal, but a combination of good writing and good acting meant she was able to transcend that. Seven was also a little overused (but can you blame them?) The obvious eye candy becoming the focus of the show understandably created tension, but what she brought to the show made it more than justifiable.