According to Earl Weaver Baseball back in the day, it might be Dave McNally. Outside of that circumstance, I'd think that Koufax would be hard to beat. Randy Johnson is one that I'd never have thought of, but definitely fits in the conversation.
I've been playing this sort of GM game since Early Weaver and Front Page Sports football in the ninties. I guess before that it was XOR football and college basketball in the 80s. Boy those were the days. Come to think of it that XOR basketball game really had a great in game experience. Better, I think than the college sims today. I guess I need to do more of that checking out.