I love Lenard as Sarek. For me, he'll always be the ultimate Vulcan (along with Spock, of course, but Spock is a class of his own). When Sarek deboards his shuttle in "Journey to Babel" it's as if Vulcan itself arrives. Ya know what I mean?
OTOH, I've always admired his subtle performance when he urges Kirk to recover his son's Katra in STIII. The way he fights back his "emotions" with effort, and his almost reluctant, yet sincere respect for Kirk when he returns Spock to Vulcan
Oh, and fine interaction between Lenard and Shatner.