Monty was a very good General. Operation Market Garden was a good idea, he just failed to listen to his recce group. Even then the operation only just fell short.
1st Airborne (The ones that held Arnhem Bridge for 9 days against the 10th SS Panzer division) had to be dropped in three stages because the Americans had taken most of the C-47s allowing them to be dropped in one. Even then the 1st Airborne held Oosterbeek and held Arnhem road bridge for 9 days expecting relief from 30 Armoured Corps on the 3rd day.
The 82nd tasked with taking Nijmegan, failed. They took the south side but had to wait until the 30 Corps arrived to take the bridge.
101st tasked with taking Son Bridge also failed. The bridge was blown and 30 corps had to stop to build a pontoon bridge.
30 Corps was also delayed in Eindhoven by the Dutch crowds. So, what should have take 4 days took 10 which resulted in the capture of 3 of the 4 intended bridges. 30 Corps being British was delayed at Eindhoven, again at Son, then again at Nijmegan the bridges that should have been captured by the Americans. Through no real fault of their own, they failed.
The 1st Airborne, the only real heros of that operation held out without supply, an airborne division against the full might of the 10th SS Panzer division for 9 days before evacuating.
This was one mistake by Monty, but not only his mistake. He was a great General. Now, Patton was a good general but very arrogant, and he didn't care about the life of his men. Rommel, don't start with Rommel, Rommel was the greatest general of the war, and he was not arrogant just confident. You cannot fault Rommel, with his little supply he achieved a lot. The only General I mark above Rommel is Heinz Guderian.
Now, I know this seems to be common thought that Stalingrad wasn't strategically important but that's a load of crap. Stalingrad was important for many reasons; 1) It carried the name of their leader, therefore its capture breaks moral. 2)It's a city, with a large garrison, you don't leave yourself open to flank attacks from a city. 3)It's a large industrial city, with huge industrial capacity, even when flattened the basis for a large industry is there. 4)It was next to the large Caucasus Oil Fields, and taking Stalingrad gave them a point of process and a garrison to hold those fields 5)Caucasus was the only viable place ot meet hte Japanese armies, although 1000 miles apart in China, German pressure on China might have caused their collapse and a meeting between the Japanese and German armies. Which would have been deadly for the Allied Armies.