For me, Kirk. I loved every actors' performance in that film, but I felt like Pine was the only one that completely nailed the essence of the character without doing an impression of the original actor. On the surface, his Kirk was very different than Shatner's kirk, but throughout the film it felt like the exact same character. I was impressed with that.
Again, though, I thought everyone did a great job. Spock was probably second for me. He pulled off angst well.