They are intimidating in their own ways, I am sure!
Tarkin has more reach and power. He won't just kill you here and there, he'll send you to prison or to be interrogated. He knows your family and loved ones. He is an arbiter above you and them the likes of which few other people in the Empire can fathom (with Vader and Sidious being above Tarkin)
Krell is physically intimidating, because of reasons already mentioned! If he wanted to he could tear you apart with his bare hands or slice you in half. But that's about the end of it, a (hopefully) quick death that leaves your friends still safe from Pong