Agree with some of the stuff that has been posted so far in the thread.I can only think of 3 styles I can't stand at the moment.
First the tumblr artstyle, it's like they are competing on who can draw a character the most unappealing. The fucking bean smile, potato noses, the melting faces, random stretch marks, unsaved legs and arm pits dyed in different colors makes me want to vomit. There much more to be said abut this style but I don't have word enough to describe my disgust and I feel I would only repeat point others already made about this topic thus far.
Ren and Stimpy legit was horrifying watching as a kid and gave me nightmares, it's something about the art style that made me feel dirty and really off putting.
Admitting I'm a huge weeb but even I can't stand the generic flat face bug eyes anime moe style.It
makes me cringe so hard I can feel my intestine doing a flip.