Market-Garden is a pretty good case: had there not been confusion among the US airborne officers at Nijmegen the bridge that was there could have been captured very early on very easily, which could (depending on German resistance beyond that) have allowed the British 30th Corps to reach Arnehm and the British airborne troops in time and might have allowed the operation to succeed. That said better planning was still needed to make the operation guaranteed to succeed.
Anzio certainly was a questionable operation because the forces sent there simply weren't enough to reach the planned objectives. In general the Italian front could have been somewhat better handled, for example the Allies may have been able to capture more German forces in Sicilia before they escaped, and they should honestly have dug in when the important objectives were reached because the Americans were already focusing on Op Overlord by this point and simply didn't have any shipping to spare to land meaningful forces further North to flank German forces. I assume not doing it would mostly save manpower and resources, though maybe they could be used in more limited but more reasonable operations.
There were opportunities to encircle German forces at Falaise earlier, capturing more veterans and officers which proved valuable for Germany in the Netherlands and likely the Bulge. Alternatively such an encirclement might have been achieved later during the Seine crossings with similar results. Could make Market-Garden more likely to succeed.
The first and third cases could accelerate the last phase of the war in Europe by allowing the Allies to get to the Rhine earlier and with possibly lower logistical issues due to more limited opposition. A successful MG would also help secure Antwerp more quickly helping with logistics and liberate most of the Netherlands by Autumn/Winter 1944 which would greatly reduce the suffering of the Dutch population OTL (no harsh winter under German control).
One could also argue that Normandy could have been better handled but it went fairly well all things considered and the Allies advanced faster than expected. But such things like a less difficult Omaha landing (better use of specialized vehicles, more accurate bombing possibly by changing landing schedules in that area or accepting a "danger close" situation) and more accurate airborne drops could help take important locations more quickly (however, a faster Normandy may end up saving the late German units from encirclement later on as they may be too far by the time the Allies break out of Normandy, so not sure how useful that would be overall).
I'm actually curious about the possibility of taking some French Channel ports earlier, before the german garrisons could dug in them.
