When I was a kid and first started watching it was Scotty, Spock, then Kirk in a relatively distant third.
As I've gotten older I have grown more and more to appreciate how Shatner utterly commanded the show. People say that about Nimoy and Spock, and of course Nimoy was absolutely wonderful, but when you consider Shatner's amazing charisma and very believable physicality (which Nimoy admitted he couldn't hope to compete with) and the well-emphasized fact (in many scripts) that Kirk was commanding a starship at a relatively young age because he was supposed to be a surpassing genius (which comes across all the better because it's typically shown, not told) . . . I'm almost sad to say it but I think Kirk has passed Spock now in my estimation. So it would be Scotty, Kirk, then Spock. Gosh, that makes me feel bad, like I'm betraying my childhood. Maybe it's Kirk and Spock in a tie for second.
Part of the issue I've started to recognize over the years is that Spock was inconsistently written. Witness the wildly different approaches to the character in Spock-in-Command Episodes. They are all over the place. By contrast, Kirk is consistently written throughout pretty much all of the series. And then of course you have S3 when Shatner and Nimoy weren't getting along or whatever the problem was, and the writers (intentionally or not) were constantly splitting them up, depriving the viewers of the awesome Kirk-Spock interaction. In my opinion, because the writers often couldn't figure out how to portray Spock in command, that separation diminished Spock, but it didn't diminish Kirk because he was more reliably depicted.
Of my top ten favorite episodes (DM, WIF, WOE, FC, UC, ROA, MM, OBS, TW, and DID), seven or eight involve Kirk and Spock working together to solve the problem for all or at least a substantial part of the episode (and almost all of them have heavy doses of Scotty). Always just on the outside of that personal list are COEOF and CHA, and of course City has the best Kirk-Spock scenes pretty much ever. CHA also features some marvelous Kirk-Spock interaction. At the end, it's Kirk, not Spock, who figures out how to destroy Nomad, but it's because Spock supplied him with the information needed to do it.
The challenge for the writers, not always met, was how to incorporate two incredibly strong and fantastically acted characters into the same script without diminishing either. But all in all they collectively had a better handle on Kirk. As the series portrayed it, Kirk isn't necessarily better with Spock on hand, but Spock is typically better with his good friend and captain nearby. This changed in the movies, and for the better.
anyway, what were we talking about... Kryten from Red Dwarf? "Ah yes, Jack Daniels brand coffee - the coffee with that smooth whiskey flavor that makes humans drunk and giddy yet reduces hangover symptoms at the same time!" 
Gotta start my Red Dwarf marathon again soon, but back to Star Trek:
