The squid's pretty scary, especially giant ones that can take down whales. But the shark's got that whole "nature's most efficient killer" thing going on. It's commonly pointed out that sharks as a species are ancient, and haven't evolved much in the course of millions of years (I'll let the science people determine if this is the case)--so the shark bears the weight of eons when its pitted against fresher ocean beasts like the squid and the octopus. That tips the scales a bit.
Octopusses (octopi?) aren't scary at all. There was this one at the zoo who would hide in a rocky corner and whenever he came out, squirted ink at his reflection. His name was Marigold.
