Elfen Lied for me. When I first watched it I enjoyed well enough and then I read the manga and I was like this is the great big mother fucking bomb of awesome. Then I rewatched the anime and then I smashed the DVD's I purchased because I was so enraged at how fucking shit is in conparison. Seriously even the voice acting for Japanese and American was terrible. Seriously who takes an awesome story cuts out huge chunks and ends halfway through, they even cut out one of the main characters.
I am going to stop now because I focus on it much longer I am going to puch a baby in the face.