Your primary targets in June 1944 would likely be Chemnitz, Leipzig, Duisburg, Stuttgart, and Madgeburg. Leipzig would be best for showing how much damage could be done, Chemnitz if you wanted to show damage and start a focus on the oil infrastructure (which the Allies did in later 1944). Essen, Munich, or Dresden would be major targets if you were going for sheer casualty numbers or for larger industrial targets. There is also a chance of the bomb being used to clear Nazi defenses in Normandy, perhaps at Omaha Beach, and having soldiers come ashore afterwards. The French will not like it, but if the Falaise pocket from OTL were used as a demonstration, German troop strength is reduced by 30,000 to 45,000 in August of 1944, there is a chance that Market-Garden actually works and that Allied troops are on the North German plain and the eastern side of the Rhine by November 1944. At that point the question is do von Rundstedt and other German military leaders decide the war is worth fighting any farther, even if they do the soldiers are demoralized further. Taking out the Wolf's Lair in July 1944 has a similar effect and probably results in a coup by army officers and politicians, it may also kick-start a civil war with the Western areas being under a pro-peace faction and Eastern areas being under a fanatical SS state.