Spock, Data, Seven, and
 T'Pol 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.
		
 
		
	 
 
I agree... and I too loved all 4 of these outsiders.  I suspect part of the reason was that, as Seven herself said to Janeway... "she" was a work in progress.  The others were, for all intents and purposes, grownups who already were "formed" when we met them, even if their friends/crews had to "tweak them" a little.
 
Seven was tweaked quite alot.
 
Kes, even though she was "growing up" on Voyager, seemed more adult than our Dear Miss Seven of Nine.
 
http://www.youtube.com/watch?v=ybGY8Ku-7kQ
 
Absolutely loved 
this entire scene from season 7's Imperfection... It and the one in Engineering with "big sister" B'Elanna are among my series favs.
 
[yt]http://www.youtube.com/watch?v=ybGY8Ku-7kQ[/yt]
 
SEVEN: Every person on this list died under your command. You accepted their deaths, but I don't believe you'll accept mine. 
JANEWAY: That's presumptuous. 
SEVEN: I'm not being egotistical. On the contrary, I'm trying to make a point. 
JANEWAY: Which is? 
SEVEN: All of these crew members came to Voyager with unique personalities, but 
I've required your constant assistance to develop my individuality. 
JANEWAY: And you've come a long way. 
SEVEN: But not far enough. I've disappointed you. 
You feel your task is incomplete. That's why my death will be difficult for you to accept. 
JANEWAY: Is 
that what you think? That you haven't lived up to my expectations? 
SEVEN: Clearly I haven't but I want you to know that the failure has been mine, not yours. 
JANEWAY: You haven't failed, Seven. You've exceeded my expectations. You've become an individual, an extraordinary individual. If I'm having trouble accepting your condition it's only because I don't want to lose a friend.
 
 
