I'd say RSC, but I didn't expect a great deal from him in the first place, he had one great season and that was it, was never ever going to be worth £18 million.
Jo would be right up there, after his exploits in Russia and what we paid for him we all expected him to be someone who would fire us into the Champions League.
Samaras would also be another - I remember at the time I was delighted we signed him after Arsenal had been linked with him and tbh despire the odd good flash, his performances never reached the quality we hope and generally looked out of his depth here.
As for longer ago ones, George Weah? Steve McManaman? High profile signing, Had a great debut against Villa but can't remembering him having another good game ever while at City. Although to be fair he was on the way down by then anyway.
Lee Bradbury?
Benjani? I know we didn't pay that much for him and he scored at the Swamp, but, when we signed him and after he scored the winner I thought he was the answer to our striking problems - he was poor from that point onwards and never looked like reaching the standards he had with Portsmouth. Another one who had one good (half) season and got a move to City on the back of it.
Bojinov promised a lot, would be harsh to put him in though due to injuries.
Robinho was a let down in the sense he didn't quite set the world alight and perform consistently to the world class standard we knew he could, but on the other hand signing him made a big point to other players, and he was still top scorer in his first season with 15 goals - not bad. But yeah he didn't quite reach the heights we dreamed of, but, considering some of the rubbish we've had, it would be a bit harsh to name him as the biggest let down.