Anthony Mundine. No amatuer career and fought for the world title in his 11th pro fight. Almost went the distance and was a close fight until the stoppage (which for some reason people still ridicule). Haseem Rahman apparently had no or a very limited amatuer career, before going on to become the undisputed heavyweight champion of the world.
S.T Gordon, who beat Trevor Berbick back in 1983, and won the WBC cruiserweight title by KOing Carlos DeLeon the same year I think.