3. Pellegrini - carried on an already successful team, played good football, some good signings and very underated outside of City.
2. Mercer/ Allison - made City a powerhouse in the late 60's/70's (as we all know) loads of trophies, great players etc. took City from div.2 to champions, was easier for a team to do this then obviously with various teams having come out of div 2 to win the top division in a fairly short amount of time.
Also City's most successful manager (management team) with our only European cup to date and clearly they both had a love for City which is always a plus.
1. Mancini - you have to remember the situation he came into, City were on a run of something like 8 draws in a row before Hughes beat Sunderland 4-3, we were in 11th (ish?) he took us to one game off the champions league that season, 3rd and an FA Cup in his second and a community shield and the league in his 3rd. Not only that he had the added pressure of knowing that City had unlimited resources so as soon as anything went wrong everyone would be on his back.
He also did that whilst controlling players like Balotelli and Tevez who were clearly not the easiest characters. Obviously the characteristics that lead to his success at City were also his downfall. But without a doubt he did as well as anyone could do with the team he was given.
(I could include myself having won every trophy possible on Championship Manager 05 with Kiki Musampa as my star player ;)