I wasn't really enthusiastic about these choices, not because they weren't good, or even great, but... I don't know, the only choice that leapt out at me that made me want to vote was Daystrom.
Lenard's great, but he's great in the same way whatever role he's playing, and I'm not as big on his first ep as others are. Montalban's a predictable crowd-pleaser too, but that's more about magnetism than great acting probably.
Daystrom just has this great, intense, unique "thing", though... I hate that I've forgotten the actor's name. He's boiling inside, being pulled in different directions at once, benevolent, malevolent, a genius, clueless... Far more interesting to me than the quiet dignity of Lenard or the broad romanticism of Montalban. You think about Daystrom afterward, far more than those other two.
Van Gelder and Zarabeth I'll second, and apparently I'm the first to nominate Michael Dunn, a favorite actor of mine, and Diana Muldaur.