My opinion of the last 20 years-ish.....
Pep - Sane (so far....)
Pellegrini - KDB (but it could have been Negredo and could end up being Fernandinho or Sterling)
Mancini - Silva (but Aguero and Yaya are pretty close IMO)
Hughes - Kompany (although he also signed Zabba, Barry and De Jong, and Robinho was a statement of intent even though it didn't really work out)
Eriksson - Elano (and I rated Corluka too)
Pearce - Hart (and Vassell was good business for the price)
Keegan - Benarbia (but Anelka, Bosvelt and James were good signings)
Royle - Goater/Dunne joint best signings (but Morrison and Bishop were great signings too)
And, honourable mention for Kevin Horlock and Gerard Wiekens, both signed by Frank Clarke IIRC.