Shatner. But I preferred Nimoy's Spock to Shatner's Kirk.
Agreed. Shatner was certainly the intended star. He was a far more popular actor than Nimoy at that point. Nimoy was supposed to be the secondary star. But as we all know, Nimoy became more popular. After all, Spock was the unusual one, the alien. And besides, Nimoy did a great job with the character. He was able to get his salary renegotiated, but Shatner was still the big money maker, as well as holding a percentage of the residuals.
Impartially, I would say they were co-stars. Both were needed. Without Spock, the series would have died off sooner. Same is true with Shatner.
Shatner was a very good actor in those days, in terms of the fact that he was thorough with his lines, punctual, and had minimal takes compared to the others. I felt his portrayal of the captain was very good... up until the 3rd season.
Nimoy just continued to improve. He actually became even better in the 3rd season.
But according to "Inside Star Trek", the 3rd season was a mess. With Roddenberry on the sidelines, and the loss of both Solow and Coon, the series suffered. One thing that Roddenberry had was good sense on script rewrites. And he'd stopped doing it. So... the actors would just do what they had to do and leave. They all knew that the series wasn't going to be renewed, as word had already spread of the 10pm Friday slot. The writing was on the wall.