All of the obvious answers have already been said, so I'll go in a little different direction with an Honorable Mention:
Randy Shannon
He was undeniably a fantastic DC, the definition of a "Miami Guy." He appeared to be a disciplinarian but was also loved by the players. I thought he would be the guy who was going to stop the slide to mediocrity.
A serious and stoic figure on the sidelines.....
But man was I wrong, he was completely clueless. Way in over his head. Simply was not qualified to successfully run a major college football program.
He hired bad OC's (Maybe except for the one who famously called him a dumb mother f***er).
I might not be remembering this correctly, but is it true that he boasted about never stepping foot in Miami Central to recruit? But I do remember watching that Wisconsin TE get what felt like 1000 receiving yards in the Champs Bowl.
And then finally, he went on to coach at both UF and FSU. Gross.
Radio might not be the biggest disappointment in (recent) Miami history, but IMO he's certainly in the conversation.