Amauri, Poulsen, Diego, Tiago, Felipe Melo and Rush (from what I've read and been told about the welshman's Juve stint), for me, are the worst Juventus flops. I'm also tempted to say Krasic (either he is one or he's almost there).
To be a real flop you have to be a player of a certain reputation, have a certain amount of hype and expectation surrounding you, and cost a certain amount of money. Where to draw the line on what that "certain" is, we can debate, but if your name is Olivier Kapo or Gladstone and came to the club on a loan deal, you couldn't be considered a real flop in my opinion.
Edit: Martinez, who I forgot about (I think my mind is trying to erase my memory of him), is definitely on the list.