I honestly can't think of anyone - though Tyson was only an underdog against Lennox Lewis to my mind. He was even a 4/1 fave against Evander in the return.
The first Holyfield fight is the one I wasn't sure about. I'm pretty sure he was the favourite against Razor though.
He was an underdog in the 1st Bowe fight anyway. I'd say George Foreman in his 1st career, other than that, maybe Wlad?