Fowler.
Got nothing against the player himself, and I quite liked him at City, but, signing him pretty much prevented us from progressing and building on a decent first season back in the Prem.
The signing of Fowler led to the resignation of Bernstein. That opened the door for Keegan to bring in more shite players like McManaman, Mills, Thatcher, etc etc. The following season we had well too many players who became has-beens at City. Also, we had Anelka who was shit hot with Fowler up front which never worked yet Keegan persisted in playing them together. (Remember Belmadi and Anelka up front against Fulham? 4-1?! Then Keegan insisted on dropping Belmadi for Fowler for the next game against West Brom and we lost 2-1?) The signing of Fowler halted our progress by 2 years and eventually led to Keegan's resignation/sacking.