Can I suggest that a new assessment of the Normandy Campaign is well worth reading, by a British military historian (John Buckley) that is not uncritical but is a necessary first-step in reassessing Montgomery as military commander and is purchasable on Amazon (Title:"Monty's Men"). Another important study is Richard Lamb's "Montgomery in Europe" (which can also be obtained through Amazon) and again is both objective and critical. But, what comes out of both these works is that Montgomery was the master of the set piece battle something that Patton for all his brilliant opportunism could never have achieved.
Reference has been made to the amount of forces confronting the Anglo-Canadians as compared to those on the American sector, and it is instructive to set them out: 5th July, British sector = 590 Panzers + 64 infantry battalions. The American sector = 215 + 63. 10th July British = 610+ 92 American = 190 + 85. Some quotes: "On July 25th the U.S. First Army launched a fresh offensive, 'Cobra', while the recently landed Patton's Third Army was ready to follow it up. The last German reserves had been thrown in to stop the British" (Basil Liddell-Hart "A History of the Second World War" p522) "Certainly if even a single Panzer division had been put in reserve, the American break-out at St. Lo (25th July-4th August) would have been far more difficult" (Richard Lamb "Montgomery in Europe p134). This, of course, was Montgomery's plan, to draw in the bulk of the German forces onto his 21st Army Group allowing Bradley to develop his plan for a break-out, which was done brilliantly.
The bottom-line is that the success in Western Europe was down to the leadership and fighting qualities of all the Allied troops, not just one particular army.