How did I not think of opening such a thread myself?
I think we should define the notion of "flop", though. A flop to me is a player on whom high expectations were placed and who turned out to be a huge disappointment. Therefore, I would not include guys like Galia, Helveg or Mirkovic in my list : these were ok players who were never asked to single-handedly lead the team to glory and who in fact played half decently.
Anyway, here are a few famous flops which should be added to the list.
Claudio Borghi : this offensive midfielder had made a big impression in the Intercontinental Cup final between Argentinos Juniors and Juventus. He was the first big buying of the Berlusconi era, as the latter saw him quite simply as the new Rivera. Sacchi did not share that opinion, prefering Rijkaard himself. But Berlusconi was determined to bring this wonderkid to Italy, and finally bought both Rijkaard and Borghi, who was sent to Como on loan, where he played about 10 games and never scored.
Luther Blissett : Milan bought him in the early 80's from Watford. He had a reputation for being an implacable striker and was expected to revive Milan's lost glory. The man scored about 3 goals in the one season he played for us. He once took a penalty that flew over the stands of the stadium. The fans renamed him "Callonissett", for his ability to dribble himself.
Ian Rush : the Welsh center forward was bought by Juve from Liverpool, where he had imposed himself as one of Europe's deadliest strikers. Him and Laudrup were expected to re-create the mythical Sivori/Charles tandem. Soon enough though they reminded Juve fans more of Cheech and Chong than anything else. Laudrup was given a second chance, while Rush was promptly sent back to England at the end of the season.
And here are a few other legendary flops from other teams :
Darko Pancev : they used to call him the "cobra", for his deadly scoring instincts. Soon enough Inter fans found another sobriquet for him : the "Lizard". Indeed, very much like the green reptile, Pancev liked to remain motionless in the penalty area, apparently taking a sunbath.
Kazu Miura : the first japanese to play in the serie A. He scored one goal for Genoa.
Hugo Maradona : Diego's brother. The "pibe" said of him : "he will become even better than me". A few anonymous appearances for Ascoli is all we remember of him.
Oscar Dertycia : he joined Fiorentina from Argentinos Juniors. He was seen as the ideal partner for Baggio. Needless to say that he was not. Borgonovo imposed himself as the Ponytail's alter-ego, scoring 15 or so goals. Therefore Borgonovo joined Milan the next season, where he went awol.