Off the top of my head, Monty gets my vote. While an overall look of his, in addition to other Allied commanders', campaign in WWII would show that he did well, there were many instances that showed him to be impatient and demanding in his dealings with units under his command. In many cases, he was stubborn, arrogant, and so was unwilling to change his plans with review of other Allied commanders. Once he thought of a plan, he would stick with it, despite any obstacles that may arise.
For example, as mentioned above, Operation: Market Garden. There were many disasters that were caused due to Montgomery. Some examples are:
1) Monty's unwillingness to adjust his plans when the German 15th Army and another large armored group were within 10 kms of the Allied drop zones and 2) Logistical plans for supplies and reinforcements weren't given enough time to be set up for such a large operation that was to take place.
While there were more factors that led to Market Garden's failure, Monty, as one of the few military leaders that had a role in planning the operation, acted stupid and arrogant when a little bit of caution could have saved hundreds, if not thousands or tens of thousands of lives in the European and African theatres.