I tend to stay away from stuff with a lot of hate centered around them so I honestly can't say I've ever actually seen a really bad anime in terms of actual quality. But I have indeed seen a few things that I didn't like just because my personal tastes differ from that of other individuals. A few examples would be DearS, Ah! My Goddess, Naruto, Green Green, Dragonaut, Bleach, and Nagasarete Airantou.
Just to name a few...