Why do a lot of people think that Hyoyeon is the ugliest member in SNSD?

Hyo doesn't fit korea's beauty standards, that's all.

she has strong proportions and her beauty is exotic, like Sooyoung's
that's why these two are usually considered the prettiest in Europe and America

I agree I think Sooyoung is the prettiest member and Im from Europe, Hyo also same just she isn't prettiest in SNSD for me, but she aren't ugly too and not the least prettiest in SNSD, while Yoona considered as the prettiest, for me is one of the least prettiest...^^

Because she has strong facial features like a wide nose
short body and un-round face.
In Korea, to be considered pretty wewe must have small facial features and a simple round face.
Weirdly enough she has european features so she is considered beautiful