I didn't include KIrk because he has a rep for overacting plus I don't think I have ever seen anyone compare these two guys so I figured I could do that. I am going for Nimoy. Not only did he have the harder character to pull off he was able to be very emotional without being overtly emotional. Something he doesn't get enough credit for was his ability to modify his voice to fit whatever emotion he wanted to convey. PLus he is good at mimicking the act of thinking. You can look at him and see that he is thinking over a problem in his mind and it goes beyond just reading his lines. I think Spock felt more like a Scientist than McCoy did as a doctor.
Jason
Jason