I would not say the best on the Normandy campaign - because that depends what your requirements are! If you're purely interested in the fighting, you're better off with Max Hastings' "Overlord" or John Keegan's 'six armies in Normandy'.
D'Este's book has a lot more about the grand strategy, including 'the war between the generals' - but a lot better written and documented than the Irving book on that subject! There is a lot of dissection of Montgomery's decisions, directives and letters, as well as post-facto arguments. And all very well documented. despite arguments in other Amazon reviews that D'Este is biased, I find him quite impartial; he reveals Monty's weaknesses, as well as Bradley's loathing for Montgomery, but also gives hono(u)rs to these two where due. He also gives the background for Montgomery's and Dempsey's caution - the very limited resources by this time, Britain coming to the end of her manpower; plus their problems with the strategic air forces; plus their *inability* to believe in American *ability* to move rapidly - once out of that wretched bocage and those swamps. Having said that, the book is mostly about the British and Canadian Army fronts, as it focuses on Montgomery; the American front is not mentioned very much.
To me, D'Este is fair; and he has done his research. My only criticisms are what comes over as excessive detail to bolster his arguments, and the maps that lack some placenamens that appear again and again in the text.