Ibrahima Bakayoko
Nii Lamptey
Freddy Adu
Sadly, Valerei Bojinov in most recent times, (co-owed by Fiorentina and Juve and United used buy him guaranteed for £28m unless you cheated and got in there first)
From the mid-90's CM games, Chris Armstrong used to be a world beater, whereas in real life at Palace he was just a good striker.
On the really old ones, like the first one I think it was, there was a player called Mukhadim Mukhadamijev who would guarantee you 30+ a season. I don't know what he was like in real life but I don't recall anyone decent buying him!