Any candidate should surely be rated on an expectation basis, which rules out a lot of these already mentioned, especially the Anderson/Plange era signings, as most were unknowns on arrival and so were expected to be pretty awful anyway.
On an expectation basis 2 stand out for me....Andy Currier, who arrived as a so-called World Class centre and left as a complete let down and Steve Roach, who arrived as the best and most feared prop in World Rugby, and departed as a massive anti-climax, only showing some improvement when Les Boyd arrived mid season and allegedly told him to shape up and stop taking the club and its fans for a ride....