I'm still trying to figure out how Joe Calzaghe ended up as an ATG, but there he arguably is and i can't see anything about him that is extraordinary. Will, heart and guts. Haugen is a worthy shout. Never understood the phenomenon that was Bazooka Limon. Looked like a slow motion prelim guy with no great physique but he just kept winning and wearing other more gifted fighters out time after time. Admirable indeed.
For me the greatest overachievers are Tommy Burns and Rocky Marciano. Look at what Tommy Burns achieved... he is ranked 4th in the list of the most consequtive world heavyweight title defences (13 times defended the title). He was a small man for heavyweight division even back in the beginning of the 20th century. He won the world title, made 13 defences, went on a world tour, wrote a book on boxing, had around 60 fights and never been KO'ed.