Zhukov, Rostossevskiiy, Konev, a few others from the eastern front. Richard O'Connor  for the British, Vandergrift and Holland Smith from the US Marines. Eichelberger, Collins, Patch,  Ridgeway for the US Army. If there were Air Force generals, nobody could touch Spaatz, Doolittle, Kearney , or the mad bomber, himself , Curtis LeMay. Dowding for the RAF and Chennault for the USAAF also deserve mention.

Too bad admirals are excluded, The USN has a galaxy of them; Spruance, the best commander of WW II, PERIOD, Clifton Sprague the second best admiral of WW II, PERIOD. Nimitz, Turner, Barbey, Lee, Oldendorf,  Kincaid, Fitch, Forest and Freddy Sherman , Mitscher, McCain, and the often ignored Charles McMorris. Many of them were better than the usual run of the mill Western admiral. The British had Frazier, Tovey, Cunningham, and Vian, all vastly overrated, but their best naval commander, a true genius, Sir Max Horton, never received the honors he truly deserved.

Neither did the Canadian, Leonard W. Murray.

Picking among the Australians is tough, Vessey, Herring, Mooreshead. I know something about these men, but I don't know enough to have a qualified opinion. Potts?, here was a Australian general I believe unfairly sacked for his successful.actions along the Kokoda trail.  

You want the worst Allied generals of the Second World War.

Famous ones? Bernard Law Montgomery.
Neville Ritchie.
Frederick Fredendall
Douglas MacArthur, when he listened to that son of a bitch, Sutherland
Grigory Kulik of the Soviet Union and Leslie McNair of the US were particularly awful though.

Herald
 

For underperforming Allied generals you might want to add Mark Clark, Herald !  

I heard the name Spaatz mentioned as among the best generals.

depends what you want, to stop allies killing each other ike, for armoured manouvere warfare patton, for attrition monty. but overall i agree with slim, little material, few men, apaalling conditions, fanatical enemy and little public support.

On the Western and Southern Fronts, Patton has to be one the best. Twice his moves busted up entire German armies. In Sicily, he had to be restrained by higher ups. If the hadn't restrained him, he would have driven all the way to Messina, taken the city, and trapped all the Germans on the Island. Instead, the allies ended up fighting those same Germans in Italy. In France, he would have been the leader of the American forces on the ground, if he hadn't been to much of bulldog. When he got to France with his 3rd army, His bold moves again had provided him a chance to trap the whole German army in the region and again he was restrained.  

Zhukov... he had difficult circumstances on more levels than most other allied generals but could still contend for best performance on results.

Can you defend that example?