No one is more qualified to answer the time-honoured question 'who was the greatest fighter?' than Bert Randolph Sugar. In ''Boxing's Greatest Fighters'', the former editor of ''The Ring'' magazine not only tells us who the greatest fighters are, but tells us the order of their greatness. Could Sugar Ray Robinson have beaten Sonny Liston? The answer, most experts agree, would be 'no'. But what if, as Bert Sugar has done here, one were to take all the boxers and 'reduce them in the mind's eye to the same height, the same weight, the same ring conditions?' The answers are very different. And while some fans may express outrage that Rocky Marciano is not in the top 10 and Marvelous Marvin Hagler is barely in the top 50, others will nod sagely when they read why Sugar selects Henry Armstrong and Willie Pep as better than just about anybody else. So, whether you read ''Boxing Greatest Fighters'' cover-to-cover, or merely flip to your favourites, be prepared, at the bell, to come out arguing. For Sugar makes 'The Sweet Science' one helluva sweet quarrel.