Rivera was good. how good was Rivera? well, as Tjodalv stated, the defense took a nosedive after he left. that's how good he was. he was probably inflated by the fact he had maybe the 2nd overall best defensive personnel in team history, but they fucked a number of teams up during his time. Angelo did some really Special person shit in that offseason before 2007, notably trading Thomas Jones for what my uncle and I have since called "a case of Gatorade". (it's stupid how many great offensive players the Bears jettisoned to the Jets). but among the dumbfuck moves was the handling of Rivera. Lovie was probably whispering in Angelo's ear they could do better with that dufus Babich at the helm for less money, I'm sure.
I call Ryan at #1 because he was a genius, Fangio at #2 based on what he did with the 49ers and what he's done the last few years, and probably Rivera at #3 for those 2005/2006 teams.
EDIT: I discovered that due to his success with the 2005 Bears, he was getting interviews for HC jobs. the former Bears regime basically cunted out over this and just chose not to re-sign him out of spite I guess.