I once had an Original Series episode on, while hoping to drift off to sleep. Complete failure, as I lay there listening for the next hour at least. I'd nominate "The Savage Curtain" as suitable for radio because everything that happens, gets verbally explained. Oh look! It's Abraham Lincoln (and he sounds more convincing than Daniel Day Lewis at least). Why it's Surak, father of all we now hold dear. And Colonel Green, who lead a genocidal war early in the 21st Century. "Spock! Help me. Spock!" Give it a try... Perhaps it's just me, knowing each and every line of dialogue that was coming.