I'm thinking Nigel Benn. He never fought a non-Brit as an amateur but beat Rod Douglas to avenge his only loss (Douglas was a multi-time ABA winner and Olympian -infact robbed against Canadian Shawn O'Sullivan in LA before O'Sullivan was robbed in final against Frank Tate), he didn't just beat Douglas he beat him up in a barnstormer en route to winning the ABA's hands down.
You know I was just looking through the list of ABA middleweight champions..... RANDY TURPIN CHRIS FINNEGAN JOHN CONTEH ALAN MINTER HEROL GRAHAM MARK KAYLOR NIGEL BENN JOE CALZAGHE CARL FROCH JAMES DEGALE GEORGE GROVES All former 165lb ABA winners. F-in brilliant stuff.
And O'Sullivan was reigning 1983 World Amateur Champ (where he beat F.Tate in final) - Douglas was top class (and in turn then, so was Benn)