Martin Landau should have been on Star Trek - think of what a great Romulan or Vulcan he would have made! Barbara Bain would have been fine, too, but Landau would have been a natural.
But Greg is absolutely right about Burgess Meredith, of course.
Landau was actually considered to play Spock, but apparently turned it down. Just as well, since he showed in an interview that he really
didn't understand the character:
"I turned down Star Trek. It would've been torturous. I would've probably died playing that role. I mean, even the thought of it now upsets me. It was the antithesis of why I became an actor. I mean, to play to play a character that Lenny (Nimoy) was better suited for, frankly, a guy who speaks in a monotone who never gets excited, never has any guilt, never has any fear, or was affected on a visceral level. Who wants to do that?"