The Tyrannosaurus Rex, or the T. Rex, is probably the most feared dinosaur that ever existed.  The movie Jurassic Park certainly gave us a visual of why this is the case.  The T. Rex is incredibly fast, strong, has huge sharp teeth and claws, and seems to always be hungry.  No other dinosaurs were safe when a T. Rex was around.  The one thing I have always questioned is why his arms were so short.  WIth his speed and powerful jaw, he would even be very dangerous without arms at all.  But, if he had longer arms were he could grab and hold his prey, that would have made him much more dangerous, plus with longer arms, maybe he would have been in a better mood :-).

Of course there are other predators out there that deserve to make the list of most feared creatures, but I cannot think of anything this day in age that compares.  Now if you were in the bottom of the ocean staring a Great White Shark in the face, that would be unpleasant.  Or maybe you turn around to a lunging Lion in an African desert.  Or maybe you are running from a hungry bear in the mountains.  No matter what animal you are facing today, none of them quite compare to the size, strength, quickness, and deadliness of the Tyrannosaurus Rex.