I think Mark Lenard really ran the table in "Journey to Babel". That's not knocking his performance as the Romulan Commander, but there's nothing really groundbreaking in playing a ruthless, but honorable enemy who becomes sympathetic because of his inner conflicts over duty and morality. Again, that's not saying it was easy, just that it's not groundbreaking. Now consider Sarek, here's a guy who's supposed to be emotionless (we know that's crappola of course and we see the emotion in him) but is estranged from his son and yet as Amanda reminds him, he's secretly proud of his son's accomplishments. He also shows a wry sense of humor and deeply loves his wife. That's an awful lot of layers for one character and in my book it's a truly amazing performance.