Gotta be Spock for me. When it was just TOS, he was popular for a lot of reasons - one of which was that he gave the social misfits in the audience (like me) someone to identify with.
Of course, as some have pointed out, the main characters are all improved by their interaction. Indeed, Kirk, Spock, and McCoy (and sometimes the others) can be seen as different aspects of one persona, or as symbolic "family members" that make up one functioning family.