TOS - Spock. McCoy was too emotional too often, which admittedly was supposed to be his role - foil to Spock's logic, but still didn't appeal. Kirk did a good job of reconciling Spock's logic and McCoy's emotion in the business at hand, but seemed to be kind of a mess in his personal life. Spock had honor, logic, and really had his stuff together.
TNG - Worf. Honor, duty, and living up to the standards that the rest of his species seemed to consider admirable but unattainable - much like Spock, in some ways. I just wish they hadn't played him as being dumb sometimes.
DS9 - Worf again, with Garak a close second. Garak was just... cool.
Voyager - Tuvok. Both because of Tim Russ's personal devotion to getting a regular role on a Star Trek series, and because Tuvok seemed annoyed most of the time - which reflected my own feelings about Voyager.
Enterprise - Tucker. "Valentine to the fans", my aching hindquarters!
Yep. Never the captain - unless you count the fact that Spock was a captain from the beginning of Star Trek II onward.