I believe I am quite afraid of Gyarados.
I mean, who wouldn't be afraid of something so large and intimidating, no pun intended, lurking around in the depths of the waters below? I do believe I could keep my fear of Garchomp at bay by staying out of caves and areas of other such terrain, however; Gyarados has been spotted in many places throughout the Pokémon universe including the ocean and bodies of fresh water. One would never even know if a skulking Gyarados lay just beneath them while having a grand time swimming.
(Well, until it decided to attack...)