Spock, Data, Seven,
were the smart-n-serious outsiders who gave us social commentary ("you humans...") and quick mental calculations. You can't have a proper space mission without that.
Of the four, and I loved them all, I got the most pure pleasure from Seven of Nine
. She could be utterly cold and analytical, but also had vulnerability and desires. The character grew as her humanity re-asserted itself. She got tutoring in social skills from a hologram-- that fell in love with her.
She looked like a million bucks, sang like an angel, and was rude to the Klingon.
She was probably the best single character in the history of STAR TREK.