It's interesting, my mom really loves German shepherds now that she has known mine and always tells me about the beautiful, well behaved GSDs she sees out and about. I have made a convert out of her- it's good for the breed to see well trained, attentive, GSD out training with their owners. It does make a difference as far as public perception.
As for most intimidating, I've found any well trained GSD, of any color variety, is going to be intimidating to most of the general public. I find a dark sable with intense eyes most impressive, but I have a soft spot for saddleback black and red, too.
The only exception would be a white GSD, I do not find them intimidating... although I will admit they are pretty dogs... also they tend to be softer temperament- overall.