(こわい) The usual word for scary.   $$$
すさまじい Scary because of its extraordinary magnitude:usually used with natural disasters:a huge typhoon, a giant tsunami, a fierce storm. Emphasizes  the degree – almost beyond measuring.   NPSITU
おっかない Same as こわい – but a tad more childish.   PPP
ろしい(おそろしい) Horrible, terrifying. Like すさまじい、 恐ろしい emphasizes the extent or force of something, rather than its appearance or fangs or whatever. He drove the car at a terrifying, terribly fast speed.  You’ll shudder when you learn the awful truth about the president!
Wait, here’s a better distinction: Use 怖い or おっかない about scary things you saw with your own eyes, but USE 恐ろしい OR すさまじい ABOUT THINGS YOU READ ABOUT / HEARD ABOUT.